[CLEANUP] Update phpdoc for all FormEngine members

Furthermore ensure the initial values are correct.
All usages of the members have been checked such that no violation can
happen if the initial values have been modified.

This patch contains no change in functionality.

Resolves: #59775
Releases: 6.3, 6.2
Change-Id: I4566849bb930535b9fef8d38dba940d6a6fd86cc
Reviewed-on: https://review.typo3.org/31272
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters