Correct some inaccuracies in the registries guide

Review Request #8486 — Created Oct. 24, 2016 and submitted — Latest diff uploaded

Information

Djblets
release-0.10.x
794e617...

Reviewers

The registries guide was slightly out of date. It did not get updated to
reference get_defaults instead of default_items from a change during
code review. In addition, the link to ItemLookupError did not use a
:py:class: annotation, and was raw text instead. Both of these issues
have been fixed.

Built the docs and viewed them.