[BUGFIX] FormEngine: Custom functions

The patch adds a .rst file to document that the "low end" functions
within FormEngine (type=user, userFunc, itemsProcFunc) now receive
different data and that this may change during the development of
version 8 again.

Additionally the patch adds a change to hand over the "parent" row
for flex field processing, so itemsProcFunc for flex fields can at
least access the parent row data if needed.

Resolves: #70132
Resolves: #70467
Releases: master
Change-Id: I7319feeec8049be0e13d32418e76d48d05e9a648
Reviewed-on: https://review.typo3.org/44542


Reviewed-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Tested-by: Morton Jonuschat <m.jonuschat@mojocode.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>