[TASK] Fix typehints in docblocks of Mapper

Resolves: #47174
Releases: 6.0, 6.1
Change-Id: Ib29e83b59f6f97e19f3123700d8971e1ea2ea7e8
Reviewed-on: https://review.typo3.org/19881
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer