[DOCS] Improve doc comments for events for modifying flex form parsing

I added and improved non-trivial method descriptions from the Changelog
descriptions here: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75158
to the source code. This also eases usage for Coders who use the code
as documentation.

Some parts of the class comments were also improved.

Releases: main
Resolves: #98298
Change-Id: Ia6902c0a1699aa652ff5e7430c6ace06e50483f2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/75662


Tested-by: core-ci <typo3@b13.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Lina Wolf <112@linawolf.de>