[meta] Check documentation for 'callback' and so forth

Problem/Motivation

Some documentation refers to 'callbacks' when referring to controllers. I'm sure there's also other Drupal 7 idiomatic language in the docblocks as well.

Learning D8 is going to be confusing enough without terminology smear.

Proposed resolution

Determine canonical names for things and consistently call those things by those names moving forward.

Find obsolete usages brought forward during porting from D7, and change that documentation. Let's do it per-module.

1. Pick a submodule.
2. Read its documentation.
3. If you find problems, create an issue about it and assign yourself to it. Make this issue its parent.
4. Change out 'callback' and other obsolete terms for their new counterparts.

Remaining tasks

User interface changes

API changes