[1.6.x] Fixed #21747 -- Added {% elif %} to template topics guide.

Thanks gcc for the suggestion.

Backport of 9e5033b302 from master
[1.6.x] Fixed #21727 -- Added some links to help clarify topics/auth/default.txt

Thanks daGrevis for the suggestion.

Backport of d35f2bfd5b from master
[1.6.x] Fixed #21768 -- Corrected TemplateView context section.

Thanks nedbatchelder for the report and claudep for the patch.

Backport of c05b2f58e7 from master