[1.6.x] Clarified that makemessages -l takes a locale name, not a language code.

Backport of 5473c36cef from master
[1.6.x] Fixed spelling mistake in file docs.

Backport of a3e88e64a4 from master
[1.6.x] Fixed #23250 -- Documented that ModelMultipleChoiceField queryset may be None.

Backport of ced3e303ca from master
[1.6.x] Fixed #21567 -- Documented how to deploy multiple sites using the sites framework.

Backport of 7b9537fb27 from master