This adds documentation to the reviewboard, reviewboard.rb_platform, and

reviewboard.signals modules. Most of these had docs already, but not in

a format that would be picked up by Sphinx.
The docstrings at the top of the files show up as summaries in the

generated list of modules. The full details show up on the given module

pages. As we go forward, we'll want to flesh out more module docstrings,

to help guide contributors and developers through the relevant parts of

the codebase.

Built the docs. Saw the details on the module pages, and the summaries on

the module listings.