[TASK] Update the Scheduler's manual

The Scheduler's manual is updated with:

* description of the new tasks provided by default
  (table and fileadmin garbage collection)
* bug tracker references change to point to Forge
* hint added in the developer's section about usage of
  the save() method

Change-Id: I25b703d56db7c525b003e62d504676cbe270841c
Resolves: #27552
Releases: 4.6
Reviewed-on: http://review.typo3.org/6199
Reviewed-by: Francois Suter
Tested-by: Francois Suter