[DOC] Update outdated specification on APIv3 watchers

Whats wrong?
-----------------

The specification on watchers is inconsistent and partially outdated:

* The workpackages group specifies a `PATCH` action on watchers that **does not work** (it is not implemented)
* There is a separate group "Watchers" which contains up-to-date documentation on adding and removing watchers from a WP
    * All of this documentation takes place in the `/api/v3/work_packages` namespace, thus **could be moved**

What does this PR do?
----------------------------

This PR

* **removes** the outdated spec for the `PATCH` action
* **moves** the up-to-date specs into the workpackage group.