[1.7.x] Improved documentation for QueryDict.

Backport of 7f4e2ef1e9 from master
[1.7.x] Fixed #22814 -- Allowed ISO-8601 [+-]hh timezone format in parse_datetime

Backport of 7beaeeed from master.
[1.7.x] Fixed #22903 -- Fixed migration generation if index_together or unique_together is removed from a model.

Backport of e0cd07ec2f from master
[1.7.x] Fixed typo in revamped forms intro doc.

Backport of 77c0a904cb from master