Added docs for changes in commit dd3a883894.

Refs #20693.
Fixed "Address already in use" from liveserver.

Our WSGIServer rewrapped the socket errors from server_bind into
WSGIServerExceptions, which is used later on to provide nicer
error messages in runserver and used by the liveserver to see if
the port is already in use. But wrapping server_bind isn't enough since
it only binds to the socket, socket.listen (which is called from
server_activate) could also raise "Address already in use".

Instead of overriding server_activate too I chose to just catch socket
errors, which seems to make more sense anyways and should be more robust
against changes in wsgiref.
Translated a test to English for consistency.

Also fixed a typo.
Stopped a test from executing queries at the module level.

Currently module level queries are executed against the real database
(specified in NAME) instead of the test database; since it is to late
to fix this for 1.6, we at least ensures stable builds. Refs #21443.
Fixed #21702 -- Added different bullet styles for nested lists.

Thanks moc at mocpa.com for the suggestion.
Fixed #21120 -- Added more explicit text on using validators and link to writing validators.

Thanks nicolas at niconomicon.net for the suggestion.
Fixed #21140 -- Added 'Using cached sessions' to the performance guide.

Thanks EvilDMP for the suggestion.
Fixed #20830 -- Clarified that Django uses a customized version of six.

Thanks glarrain for the suggestion.
Fixed #14028 - Added validation for clashing db_columns.

Thanks akaariai for the suggestion.
Fixed #21070 -- Added a more complex database configuration example.

Thanks joejasinski for the suggestion.
Fixed #21001 -- Added a regression test for empty string exclusion on Oracle.

The issue was present in 1.4.x but has been reported to be fixed in master.

Thanks to @timgraham for the patch.
Fixed test suite bisection on Python 3.

Thanks to @marfire for spotting this.
Removed implication that six is part of Python stdlib.
Removed extra p (topppings->toppings)
Removed double import in mysql base.py
Isolated select_for_update tests a bit more.

This change prevents including the multiple_database test models without
duplicating the router code (we probably should do this at one point).

Refs #21148
Fixed #21151 -- Bad cleanup in schema M2M repoint test
Reformatted some GIS distance tests
Properly marked skipped GIS tests as skipped
Marked PermissionsMixin.user_permissions help_text for translation