Documentation -- Corrected error about Model.full_clean()

Although the ModelForm validation code was changed to call
Model.full_clean(), the documentation still said otherwise. The
offending phrase was removed.
Merge pull request #1502 from marfire/doc_modelvalid

Documentation -- Corrected error about Model.full_clean()
Fixed #20955 -- select_related regression

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.
Moved translator comment just above the target string
Made description of LANGUAGE_CODE setting more clear.
Fixed #20956 -- Removed useless check in django.db.utils
Fixed #20944 -- Removed inaccurate statement about View.dispatch().
Documentation -- Clarified use of 'view' in test client introduction.
Revert "Fixed #20956 -- Removed useless check in django.db.utils"

This reverts commit ce0e86cf761dc182ae6fb8c0d68964955d19ec75.

The check is necessary if 'ENGINE' is an empty string.
Thanks apollo13 for pointing this out.
Fixed model.__eq__ and __hash__ for no pk value cases

The __eq__ method now considers two instances without primary key value
equal only when they have same id(). The __hash__ method raises
TypeError for no primary key case.

Fixed #18864, fixed #18250

Thanks to Tim Graham for docs review.
Fixed #20934 -- Avoided NoReverseMatch in ModelAdmin.changelist_view

The view tried to display links to a ModelAdmin's change_view, which
resulted in NoReverseMatches if get_urls was overridden to remove the
corresponding url.
Fixed #19303 -- Fixed ModelAdmin.formfield_overrides on fields with choices
Fixed #20922 -- Allowed customizing the serializer used by contrib.sessions

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.
Typos introduced in 297f5af222.
Merge remote-tracking branch 'upstream/master'