[TASK] Make comments in DataHandler real docblocks

Change-Id: I8ae556930bd378a3756edbaa9ab86530f159f0da
Relases: 6.2
Resolves: #49784
Reviewed-on: https://review.typo3.org/22119
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters