Stabilise `django-extra` (`S610`) for release 0.5 (#12029)
The motivation for this rule is solid; it's been in preview for a long
time; the implementation and tests seem sound; there are no open issues
regarding it, and as far as I can tell there never have been any.
The only issue I see is that the docs don't really describe the rule
accurately right now; I fix that in this PR.
