With currentmodule:: set properly, none are needed. Extra "machine." prefix
produces wrong indexing data.

Class designator will be used as is in indexes, so must match actual class
name.

Makes sure that classes described in these separate files are properly
designated as belonging to "machine" module in indexes.

This makes all common files "port-aware" using the .. only directive.

Also includes documentation about minimum baudrate.

Addresses issue #1090.

Addresses issue #950.

The inline docs (prefixed with /// in .c files) have been converted to
RST format and put in the docs subdirectory.