[DOCS] Update outdated information about FlashMessagesViewHelper caching

The ViewHelper does not clear cache since 7.3.0 change:
https://review.typo3.org/c/Packages/TYPO3.CMS/+/34818
Still the documentation states it will clear cache.

Instead users need to take care to properly clear cache.
The background:

FlashMessages often tend to be part of a process and already work with
proper cHash and URLs. Unique URLs often lead to unique output of
FlashMessages.
Under some circumstances flash messages might be created before
redirecting to a generic URL.

Resolves: #93636
Releases: master, 10.4
Change-Id: I15479d31671a04ce12336583767277f0fc25cbca
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/68173


Tested-by: Mathias Brodala <mbrodala@pagemachine.de>
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Mathias Brodala <mbrodala@pagemachine.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>