[DOCS] Add hint to HtmlViewHelper about avoiding usage in backend context

The docblock of the class is rendered for documentation, see:
https://docs.typo3.org/other/typo3/view-helper-reference/main/en-us/typo3/fluid/latest/Format/Html.html

This information should therefore be mentioned.

Related: #99031
Resolves: #99536
Resolves: https://github.com/TYPO3-Documentation/Changelog-To-Doc/issues/250
Releases: main, 11.5
Change-Id: I6f21710975a8916e27cdcd38c0bcb83a129319ea
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77436


Tested-by: core-ci <typo3@b13.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>