These classes are pretty important, but we didn't have any real class

reference documentation for the attributes. This change introduces

documentation for these, fleshes out some of the docs for methods, and

updates one of the attributes to be a documentable @cached_property

(using Djblets's new, smarter version).
Something worth noting is that Sphinx does not provide documentation

inheritance for attribute docs, so the Review Board documentation still

won't have those documented, but at least we have a good reference now

in Djblets.

Built the docs and looked through them. Everything was documented.