Document the behaviour when deleting or removing a user

Need to document what happens to author attribution when a user is deleted or when a user is removed from a site within a Multisite network.

* Is the user still attributed?
- User deleted: no
- Multisite user removed: yes (needs verifying)
* Can the user still edit posts they are attributed to?
- User deleted: nope
- Multisite user removed: no
* Does the plugin remove the attribution connections in the database?
- No
* Does the plugin gracefully skip attribution connections that use user IDs that don't exist?
- It should but this needs verifying, for example when editing a post that is attributed to a user who has subsequently been deleted

A future task will cover improving the UX when deleting or removing a user, for example by presenting the option to reassign or remove attribution for the user.