Improve documentation of optional arguments to callables

[davidorme]

Is your feature request related to a problem? Please describe.

As @j-emberton pointed out over on #267, the docstrings don't do a consistent job of making it clear where arguments are optional and what happens when they aren't provided. It's clear which arguments are optional from the typing, but then individual argument descriptions should consistently end with something like "Optional, defaulting to XYZ".

Describe the solution you'd like
Review the signatures and consistently note the optional arguments in the Args section of the docstrings.