[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.