Add documentation for recent changes.
Add documentation for recent changes.

pombredanne/pex,sixninetynine/pex,pantsbuild/pex,pantsbuild/pex,rouge8/pex,jamesbroadhead/pex,kwlzn/pex,sixninetynine/pex,mzdanieltest/pex,jsirois/pex,pombredanne/pex,snyaggarwal/pex,jsirois/pex,jamesbroadhead/pex,URXtech/pex,kwlzn/pex,mzdanieltest/pex,URXtech/pex,snyaggarwal/pex,rouge8/pex