Fix issues and add missing content to the codebase docs.
Review Request #11122 — Created Aug. 6, 2020 and submitted
This fixes up some warnings found when building the codebase docs, which
were largely syntax-related or compatibility issues with modern Sphinx
(such as the case sensitivity of terms in the glossary).
It also adds some missing module docstrings and some missing modules,
and updates our Django class references to reference Django 3.1.
Built the docs with the latest version of Sphinx.
Verified there were no errors.
Checked the generated docs to make sure my fixes were applied.