[1.6.x] Fixed small typo in django-admin.txt

The word 'by' seemed to be missing from the first line of the compilemessages command.

Backport of 5ca522b9ed from master
[1.6.x] Fixed #21553 -- Ensured unusable database connections get closed.

Backport of 5f2f47f from master
[1.6.x] Fixed #22343 -- Disallowed select_for_update in autocommit mode

The ticket was originally about two failing tests, which are
fixed by putting their queries in transactions.

Thanks Tim Graham for the report, Aymeric Augustin for the fix,
and Simon Charette, Tim Graham & Loïc Bistuer for review.

Backport of b990df1d63 from master
[1.6.x] Documentation fixes for the select_for_update change.

Refs #22343; thanks Tim Graham for the fixes.