[1.6.x] Fixed #21726 -- Clarified that password should not be included in REQUIRED_FIELDS.

Thanks russellm for the report.

Backport of 6f06c749b7 from master
[1.6.x] Fixed #21529 -- Noted that {% url %} encodes its output (refs #13260).

Backport of dfc092622e from master
[1.6.x] Added a link to the 1.6 release notes which also fixed a rendering issue.

Backport of 2171341162 from master
[1.6.x] Fixed #20834 -- Described how caching of user permissions works.

Thanks Giggaflop and Jennifer Casavantes.

Backport of 5f9790072d from master
[1.6.x] Don't show `self` in the list of arguments of a method.

This is consistent with Python's official documentation
and it's a sphinx recommendation too[1].

[1] http://sphinx-doc.org/markup/desc.html#dir-method

Refs #21855.

Backport of 79e1d6ebd70898d514a44b85648e3d24104c4243 from master.
[1.6.x] Always use parentheses when documenting a method with no arguments.

Backport of 05d36dc06e6d767bb28993c65a54b703f319a386 from master.