[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>

typo3/sysext/core/Documentation/Changelog/9.5.x/Important-89269-IntroduceUpgradeWizardForInvalidBackendUserConfiguration.rst

+.. include:: ../../Includes.txt
+
+===================================================================================
+Important: #89269 - Introduce Upgrade Wizard for invalid Backend User configuration
+===================================================================================
+
+See :issue:`89269`
+
+Description
+===========
+
+Each database record in table `be_users` stores an individual user configuration in
+the field `uc`. It is exposed during runtime in :php:`$GLOBALS['BE_USER']->uc`.
+
+In older TYPO3 versions serialized class instances of :php:`\stdClass` have been
+persisted to mentioned field - which is not possible anymore since TYPO3 v9.
+
+A corresponding upgrade wizard **"Update backend user configuration array"**
+aims to convert :php:`\stdClass` data to plain PHP arrays using scalar values.
+
+Since this upgrade wizard was introduced late with TYPO3 v9.5.11 it is suggested
+to make sure this upgrade step is executed in case a website has been updated
+before (e.g. from TYPO3 v8 to TYPO3 v9.5.1).
+
+.. index:: Backend, Database, ext:install