Improve documentation for users app

[Samwalton9]

Updated and expanded documentation for the users app. This PR also includes some minor tidying of unused variables.

Where changes were more than documentation changes or might have been confusing I made individual commits to aid with review.

Phabricator Ticket

T264425

How Has This Been Tested?

Tests passed, and did some clicking around locally. I tested the Authorization admin page because I was especially unsure about that code removal.

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

[katydidnot]

Thanks for your work updating documentation, this will be very helpful for me and other new team members.

I had one minor nit, but feel free to ignore.

[katydidnot]

minor nit: QuerySet instead of Queryset for consistency.

[suecarmol]

Thanks for all of your work adding more documentation in key places in the library. I'll go ahead and merge this once I commit the suggestions below 😄