Use `LOOKUP_SEP` in `get_or_create`.
Fixed #20282 -- Typo in WSGI docs.
Fixed #20286 - Typo in static files docs.

Thanks bmispelon.
Fixed #20285 - Added missing commas in translation docs.

Thanks cody.j.b.scott@
Complemented documentation following commit be9ae693c

Refs #17840. Thanks Carl Meyer for noticing the omission.
Explained the pattern for special methods compatibility.
Fixed #20276 -- Implemented __bool__ for MergeDict

MergeDict evaluates now to False if all contained dicts are empty.
Thanks til for the report and the initial patch.
Fix != operations on lazy objects.
Updated tutorial INSTALLED_APPS section (removed contrib.sites)
Fixed #20104 -- Changed VersionDirective in order to avoid ambiguity.

As explained in ticket #20104, the use of versionchanged/versionadded
was confusing.

To solve this ambiguity these directives no longer accept a second
argument but now they only receive the version number (1st arg) and then
a content with the proper comment.
Removed unused imports.
Adapted uses of versionchanged/versionadded to the new form.

Refs #20104.