[1.6.x] Added missing parameter in npgettext_lazy docs

Backport of 413307fe35 from master
[1.6.x] Fixed docs for EmailMessage extra_headers attribute

The EmailMessage documentation describes the initialization parameters
and states they 'can be set at any time prior to calling the send()
method.'. However, the 'headers' parameter's corresponding attribute is
called 'extra_headers'.

Backport of cab72a439b from master
[1.6.x] Noted that .clear() will delete intermediary models

Backport of a8e84521c4 from master
[1.6.x] Fixed versionadded misuse in the admin documentation.

Backport of c82e850eed2eb3957ed792908b73622ebd3034a7 from master.
[1.6.x] Fixed spelling of compatibility.
[1.6.x] Fixed #21398 -- Fixed BCryptSHA256PasswordHasher with py-bcrypt and Python 3.

Thanks arjan at anymore.nl for the report.

Backport of d15985d81f from master
[1.6.x] Fixed #21372 -- Corrected docs regarding translating LANGUAGES.

Corrected LANGUAGES documentation on how to translate language
names. Now using django.utils.translation.ugettext_lazy instead
of a dummy gettext() function.

Thanks to Salvatore for the report.

Backport of 8bc350b38516d8c3a14aed113dd3402b9375b75c from master.
[1.6.x] Removed a mention of `Form._errors` from the documentation.

Also removed a sentence that was incorrect: raising a
`ValidationError` inside `Form.clean` doesn't clear the
`cleaned_data` attribute.

Thanks to loic84 and timograham for the review.

Backport of 9aa6d4bdb6618ba4f17acc7b7c0d1462d6cbc718 and
0048ed77c7016a03aed6dbc255a6372f3df2ccd5 from master.
[1.6.x] Fixed #21410 -- prefetch_related() for ForeignKeys with related_name='+'

Regression introduced by commit 9777442.

Thanks to trac username troygrosfield for the report and test case.

Backpatch of cb83448891f2920c926f61826d8eae4051a3d8f2 from master.

Conflicts:

	tests/prefetch_related/models.py
[1.6.x] Added release note for #21410; thanks Loic.

Backport of 94d567ba30 from master