[1.5.x] Fixed #20654 -- Fixed type of `m2m_changed`'s `pk_set` arg in docs

Backport of 9a2b07f1b4 from master
[1.5.x] Fixed #20665 -- Missing backslash in sitemaps documentation

Backport of 5005303ae7919eef26dab9f8ba279696966ebf1d from master.
[1.5.x] Fixed typo in docs/ref/signals.txt

Backport of 6fcb7ba842 from master
[1.5.x] Added missing preposition in documentation

Backport of 2ec54e7fbc from master
Fixed #20670 -- Fixed project layout in reusable apps tutorial.

Backport of 70d3adf9f2ebfdfc9e64376b71b596ebb12d77f8 from master.
Thanks to nicklasp for the report.
[1.5.x] Fixed missing slash in reusable apps tutorial.

Backport of 5caced89e0ac2f942b68bc5f163d156a42880f16 from master.
[1.5.x] Fixed #20677 - Typos in generic_inlineformset_factory docs.

Thanks Riley Strong for the report.

Backport of 3fd0ee5b46 from master
[1.5.x] Fixed #20691 -- Added a note for __str__() vs. __unicode__() and Python 3.

Thanks garrison for the suggestion.

Backport of 8a679386c3 from master.
[1.5.x] Fixed #20687 -- Added documentation for django.core.signing API.

Thanks Baptiste Mispelon for the suggestion.

Backport of c5bc98d7e1 from master.
[1.5.x] Fixed #20695 -- Fixed contrib.humanize translation syntax errors

Affected language: Mexican Spanish, Mongolian, Romanian, Turkish
Translations for 1.6 and master will be refetched from Transifex.
Thanks ruless at gmail.com for the report.
[1.5.x] Fixed #20609 -- Documented how to use request.user with RequestFactory

Thanks michel@ for the suggestion.

Backport of 75041d5ea3 from stable/1.5.x.
[1.5.x] Fixed #20690 -- Mentioned LOCALE_PATHS earlier in translation docs.

Thanks rene@ for the suggestion.

Backport of 231e31c690 from master.
[1.5.x] Fixed #12579 -- Noted QuerySet.get_or_create() depends on database unique constraints.

Thanks timmolendijk, jdunck, vijay_shanker, and loic84.

Backport of 428de2e339 from master.
[1.5.x] Fixed #20673 -- Clarified that HttpRequest.user uses AUTH_USER_MODEL.

Thanks littlepig for the report.

Backport of f407f75aae from master.
[1.5.x] Fixed #20134 -- Correct list of fields that UserManager requires.

Thanks semenov and pegler.

Backport of da5069f68f from master.
[1.5.x] Fixed #19940 -- Made test.runner.setup_databases properly handle aliases for defau

Thanks simonpercivall.

Backport of 2cbd579efe from master.
[1.5.x] Fixed #19539 -- Updated custom model fields example for Python 3.

Thanks astorije@ for the report.

Backport of b9fceadfd4 from master.
[1.5.x] Fixed #14006 -- Documented that Field's 'description' attribute is interpolated with field.__dict__

Thanks abeld for the suggestion.

Backport of 2d96e660f9 from master.
[1.5.x] Fixed #20711 -- Fixed broken link in timesince.py docstring

Backport of d63327d843 from master.
[1.5.x] Fixed #12346 -- Added a note on how to validate InlineFormSets.

Thanks johnsmith for the suggestion.

Backport of 181f63c22d from master