Incomplete documentation of internal modules

[Flix6x]

This page (which index.html links to at the bottom) says it documents all internal modules, but in fact only documents a few.

[nhoening]

We'd have to list each submodule we want to document. This tedious task is now supported by a Sphinx extension: https://sphinx-rtd-tutorial.readthedocs.io/en/latest/build-the-docs.html#generating-documentation-from-docstrings