Fixed #23075 -- Added documentation on novalidate attribute and made it default for admin

Thanks to sehmaschine for the report and to Tim Graham for the review.
Added words to spelling_wordlist.
Fixed spelling mistake in file docs.
Fixed #23362 -- Clarified "database columns as attributes" in tutorial.
Fixed #17719 -- Documented that template syntax sequences cannot be used as string literals.
Allowed skipIf/UnlessDBFeature to accept several feature strings
Removed some more hardcoded backends in GIS tests

Refs #22632. Thanks Tim Graham for the review.
Fixed #19312 -- Documented MySQL TIMESTAMP columns cannot be used with USE_TZ=True.
Fixed #17707, #20734 -- Added examples to URL namespacing docs.

Thanks Daniele Procida for review.
Fixed #23250 -- Documented that ModelMultipleChoiceField queryset may be None.
Added exception name in debug error message

This can help when some exception has no error message.
Fixed #21567 -- Documented how to deploy multiple sites using the sites framework.
Fixed #22620 -- Emphasized role of DATABASE_ROTERS in multi-db docs.

Thanks Mike O'Connor for the suggestions.
Lowered memory consumption in debug ouput test
Fixed clone URL of Django git repository.