[1.5.x] Fixed a minor spelling mistake in the queryset documentation

Backport of d258cce482 from master
[1.5.x] Fixed #20165 - Updated testing example to use django.test.TestCase.

Thanks Lorin Hochstein.

Backport of 84d8b247d2 from master.
[1.5.x] Fixed #19712 - Clarified admindocs regarding which attributes appear.

Thanks Daniele Procida for the report.

Backport of b5b63079bd from master
[1.5.x] Fixed #20421 - Typo in class-based view docs.

Backport of 289afc3156 from master
[1.5.x] Fixed #20335 - Documented the {% language %} template tag.

Thanks bmispelon for the suggestion and djangsters for the patch.

Backport of 96cabba808 from master
[1.5.x] Fixed #20446 -- Documentation for SmallIntegerField does not clarify 'small'

Backport of 56e2f6ccae from master
[1.5.x] Fixed #20284 - Added a note about DEBUG in static file docs.

Thanks wim@ for the suggestion and Nicolas Noé for the patch.

Backport of 2c84f4434c from master
[1.5.X] Fixed #20183 - Clarified docs assumption that certain objects exist in database.

Thanks Tomasz Jaskowski for the patch.

Backport of 41bea033b7 from master
[1.5.x] Added some links in /docs/intro/overview.txt

Thanks Claes Ström for the patch.

Backport of bdde7feb26 from master.
[1.5.x] Fixed typo in redirect view docs

Backport of 428875775c from master
[1.5.x] Fixed #20278 -- ensured .get() exceptions do not recurse infinitely

A regression caused by d5b93d3281fe93cbef5de84a52 made .get() error
reporting recurse infinitely on certain rare conditions. Fixed this by
not trying to print the given lookup kwargs.

Backpatch of 266c0bb23e9d64c47ace4d162e582febd5a1e336
[1.5.x] Tweaked unit test 'quick start' explanation.

Thanks Jeremy Dunck.

Backport of 490672f057 from master
[1.5.x] Fixed prefetch_related + pickle regressions

There were a couple of regressions related to field pickling. The
regressions were introduced by QuerySet._known_related_objects caching.

The regressions aren't present in master, the fix was likely in
f403653cf146384946e5c879ad2a351768ebc226.

Fixed #20157, fixed #20257. Also made QuerySets with model=None
picklable.
[1.5.x] Fixed #20212 - __reduce__ should only be defined for Py3+.
[1.5.x] Imported copyreg from six.moves
[1.5.x] Fixed #20476 -- Typo.

Backport of 5d16456.
[1.5.x] Fixed #17048 - Added docs for upgrading Django.

Thanks Donald Stufft for the suggestion and
Susan Tan and Brian Fabian Crain for the patch.

Backport of ef8a9bc7fa from master.
[1.5.x] Added a note about pip upgrade flag.

Thanks Jannis Leidel for the suggestion.

Backport of 0e45f75112 from master
[1.5.x] Added some text I meant to add in the last commit.

Backport of 2699219c23 from master
[1.5.x] Tweaked a reST construct in template API docs. Refs #20104.