[TASK] Improve description of MM mappings

Mapping MM relations and proper usage of the MM-related
properties is not trivial. Hopefully these added explanations
will help.

Resolves: #59214
Releases: 2.3
Change-Id: I0eb0dcf52b90d770561f62d9d81eb7bf9113b018
Reviewed-on: https://review.typo3.org/30507
Reviewed-by: Francois Suter
Tested-by: Francois Suter