migrate alter plugin info hooks are undocumented

Problem/Motivation

The migrate module invents several plugin types:

- migration
- source
- destination
- process
- id_map

As with all plugin types, these each have a corresponding alter info hook.

However, only hook_migration_plugins_alter() -- the alter hook for migration plugins -- is documented in migrate.api.php.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet