* Implement ability to limit module documentation building:
- Added new option to plugin_formatter.py to support passing-in a list of
modules for which the documentation should be built.
- Updated docuemtnation Makefile to allow specifying list of modules via
environment variables (defaulting to all modules).
- Update instructions for building documentation and module development to
include commands and description of limiting module documentation builds.
* Updated implementation for limiting module documentation building:
- Pass list of modules (or None) to list_modules function instead of string.
- Move conversion of module list to argument parsing code.
- No special keywords. Default ("") means build all modules. For no modules just
specify non-existing module name.
- Updated documentation to reflect the changes.
* Updated implementation for limiting module documentation building:
- Use better default value, and don't treat "" as special case.
- Conditionally invoke different variants of command in Makefile instead of
using special value "".
* Minor edits
Wording tweak
