Add thorough documentation for the djblets_utils template tags.

Review Request #7738 — Created Oct. 25, 2015 and submitted — Latest diff uploaded

Information

Djblets
release-0.9.x

Reviewers

djblets_utils is one of the oldest modules in Djblets, and its
documentation was really lacking. This change fleshes it out, bringing
it up to current standards, and adds examples showing how to use all the
tags.

This started off as some cleanups to appease Sphinx, but ended up
becoming a complete rewrite of the docs.

Built the docs and read through them.