[TASK] Add missing documentation for BackendUserConfigurationUpdate

Resolves: #91417
Releases: master, 9.5
Change-Id: I690cf19965310cdb8612dca3b34f751aafb4c550
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64502


Reviewed-by: Susanne Moog <look@susi.dev>
Reviewed-by: Daniel Siepmann <coding@daniel-siepmann.de>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Tested-by: Daniel Siepmann <coding@daniel-siepmann.de>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Tested-by: Oliver Hader <oliver.hader@typo3.org>