Fixed #23982 -- Added doc note on generating Python 2/3 cross-compatible migrations.

Fixes #23982, and reverts the auto-conversion of `related_name` added in #23455.
etc.
(also not sure if it's intentional, but your line breaks could be closer to 79 characters for consistency with the rest of the docs -- noticed this on one of your other patches too)
otherwise LGTM.