Fix #1891: monkeypatch autodoc to avoid directive name conflict

[pushfoo]

Changes

Fixes #1891 , unblocks #1890

1. Monkeypatch instead of attempting to add a ClassDocumenter which conflicts with the autoclass directive
2. Add doc explaining why / how this is necessary
3. Add some whitespace

Follow up work

We should really consider making those "Is awesome Sphinx!" mugs.

How to test

Option 1: Locally

1. ./make.py clean
2. ./make.py html
3. If the following warning no longer shows up, it's good:

   WARNING: while setting up extension conf.py: directive 'autoclass' is already registered, it will be overridden
   

Option 2: CI

1. Rebase something which failed off this branch
2. Push it to GitHub to trigger CI