Use the modern Beanbag doc processor for Djblets, and fix doc errors.

Review Request #9892 — Created April 25, 2018 and submitted — Latest diff uploaded


The Djblets codebase was still using the upstream Google doc format
found in Sphinx's Napolean processor, which doesn't offer the advantages
that the Beanbag processor uses. This change moves us to using the new
Beanbag one.

Due to a bug in the Beanbag processor, though, parts of our codebase
docs were failing. The processing code wasn't supporting arguments in
the form of:

varname (type): description

While this is being fixed in beanbag-docutils, it was a good opportunity
to clean up those parts of our codebase, bringing them in line with how
we're writing the docs now, and fixing up some bad type references and
missing descriptions within the same files.

Built the docs successfully. Checked for errors in affected code, and
checked that new doc features were taking effect.