[TASK] Update doc comment for ActionController::getErrorFlashMessage

The function getErrorFlashMessage() in ActionController can either
return a string or a boolean. This patch adds bool as allowed
return type in the doc comment.

Resolves: #96697
Releases: main, 11.5
Change-Id: Ibd9bce942280c48fd82960aeaa9b6f691b6730e2
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/73232


Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>