docs/uctypes: Improve documentation.

Seealso and Limitations sectiosn added, better formatting and grammar.

docs/library/uctypes.rst

-:mod:`uctypes` -- access C structures
-=====================================
+:mod:`uctypes` -- access binary data in a structured way
+========================================================
 
 .. module:: uctypes
-   :synopsis: access C structures
+   :synopsis: access binary data in a structured way
 
 This module implements "foreign data interface" for MicroPython. The idea
-behind it is similar to CPython's ``ctypes`` modules, but actual API is
-different, streamlined and optimized for small size.
+behind it is similar to CPython's ``ctypes`` modules, but the actual API is
+different, streamlined and optimized for small size. The basic idea of the
+module is to define data structure layout with about the same power as the
+C language allows, and the access it using familiar dot-syntax to reference
+sub-fields.
+
+.. seealso::
+
+    Module :mod:`ustruct`
+        Standard Python way to access binary data structures (doesn't scale
+        well to large and complex structures).
 
 Defining structure layout
 -------------------------
 
 Structure layout is defined by a "descriptor" - a Python dictionary which
 encodes field names as keys and other properties required to access them as
-an associated values. Currently, uctypes requires explicit specification of
-offsets for each field. Offset are given in bytes from structure start.
+associated values. Currently, uctypes requires explicit specification of
+offsets for each field. Offset are given in bytes from a structure start.
 
 Following are encoding examples for various field types:
 
-   Scalar types::
+* Scalar types::
 
     "field_name": uctypes.UINT32 | 0
 
   in other words, value is scalar type identifier ORed with field offset
   (in bytes) from the start of the structure.
 
-   Recursive structures::
+* Recursive structures::
 
     "sub": (2, {
         "b0": uctypes.UINT8 | 0,
@@ -36,7 +45,7 @@ Following are encoding examples for various field types:
   a structure descriptor dictionary (note: offsets in recursive descriptors
   are relative to a structure it defines).
 
-   Arrays of primitive types::
+* Arrays of primitive types::
 
       "arr": (uctypes.ARRAY | 0, uctypes.UINT8 | 2),
 
@@ -44,7 +53,7 @@ Following are encoding examples for various field types:
   with offset, and second is scalar element type ORed number of elements
   in array.
 
-   Arrays of aggregate types::
+* Arrays of aggregate types::
 
     "arr2": (uctypes.ARRAY | 0, 2, {"b": uctypes.UINT8 | 0}),
 
@@ -52,21 +61,21 @@ Following are encoding examples for various field types:
   with offset, second is a number of elements in array, and third is
   descriptor of element type.
 
-   Pointer to a primitive type::
+* Pointer to a primitive type::
 
     "ptr": (uctypes.PTR | 0, uctypes.UINT8),
 
   i.e. value is a 2-tuple, first element of which is PTR flag ORed
   with offset, and second is scalar element type.
 
-   Pointer to aggregate type::
+* Pointer to an aggregate type::
 
     "ptr2": (uctypes.PTR | 0, {"b": uctypes.UINT8 | 0}),
 
   i.e. value is a 2-tuple, first element of which is PTR flag ORed
   with offset, second is descriptor of type pointed to.
 
-   Bitfields::
+* Bitfields::
 
     "bitf0": uctypes.BFUINT16 | 0 | 0 << uctypes.BF_POS | 8 << uctypes.BF_LEN,
 
@@ -103,17 +112,18 @@ Module contents
 
 .. data:: LITTLE_ENDIAN
 
-   Little-endian packed structure. (Packed means that every field occupies
-   exactly as many bytes as defined in the descriptor, i.e. alignment is 1).
+   Layout type for a little-endian packed structure. (Packed means that every
+   field occupies exactly as many bytes as defined in the descriptor, i.e.
+   the alignment is 1).
 
 .. data:: BIG_ENDIAN
 
-   Big-endian packed structure.
+   Layour type for a big-endian packed structure.
 
 .. data:: NATIVE
 
-   Native structure - with data endianness and alignment conforming to
-   the ABI of the system on which MicroPython runs.
+   Layout type for a native structure - with data endianness and alignment
+   conforming to the ABI of the system on which MicroPython runs.
 
 .. function:: sizeof(struct)
 
@@ -145,33 +155,55 @@ Structure descriptors and instantiating structure objects
 
 Given a structure descriptor dictionary and its layout type, you can
 instantiate a specific structure instance at a given memory address
-using uctypes.struct() constructor. Memory address usually comes from
+using :class:`uctypes.struct()` constructor. Memory address usually comes from
 following sources:
 
 * Predefined address, when accessing hardware registers on a baremetal
   system. Lookup these addresses in datasheet for a particular MCU/SoC.
-* As return value from a call to some FFI (Foreign Function Interface)
+* As a return value from a call to some FFI (Foreign Function Interface)
   function.
-* From uctypes.addressof(), when you want to pass arguments to FFI
+* From uctypes.addressof(), when you want to pass arguments to an FFI
   function, or alternatively, to access some data for I/O (for example,
-  data read from file or network socket).
+  data read from a file or network socket).
 
 Structure objects
 -----------------
 
 Structure objects allow accessing individual fields using standard dot
-notation: ``my_struct.field1``. If a field is of scalar type, getting
-it will produce primitive value (Python integer or float) corresponding
-to value contained in a field. Scalar field can also be assigned to.
+notation: ``my_struct.substruct1.field1``. If a field is of scalar type,
+getting it will produce a primitive value (Python integer or float)
+corresponding to the value contained in a field. A scalar field can also
+be assigned to.
 
 If a field is an array, its individual elements can be accessed with
-standard subscript operator - both read and assigned to.
+the standard subscript operator ``[]`` - both read and assigned to.
 
 If a field is a pointer, it can be dereferenced using ``[0]`` syntax
 (corresponding to C ``*`` operator, though ``[0]`` works in C too).
-Subscripting pointer with other integer values but 0 are supported too,
+Subscripting a pointer with other integer values but 0 are supported too,
 with the same semantics as in C.
 
 Summing up, accessing structure fields generally follows C syntax,
-except for pointer derefence, you need to use ``[0]`` operator instead
-of ``*``.
+except for pointer derefence, when you need to use ``[0]`` operator
+instead of ``*``.
+
+Limitations
+-----------
+
+Accessing non-scalar fields leads to allocation of intermediate objects
+to represent them. This means that special care should be taken to
+layout a structure which needs to be accessed when memory allocation
+is disabled (e.g. from an interrupt). The recommendations are:
+
+* Avoid nested structures. For example, instead of
+  ``mcu_registers.peripheral_a.register1``, define separate layout
+  descriptors for each peripheral, to be accessed as
+  ``peripheral_a.register1``.
+* Avoid other non-scalar data, like array. For example, instead of
+  ``peripheral_a.register[0]`` use ``peripheral_a.register0``.
+
+Note that these recommendations will lead to decreased readability
+and conciseness of layouts, so they should be used only if the need
+to access structure fields without allocation is anticipated (it's
+even possible to define 2 parallel layouts - one for normal usage,
+and a restricted one to use when memory allocation is prohibited).