[DOCS] Improve documentation about change of tt_content fields configuration

In #92659 the tt_content fields `imagewidth` and `imageheight` have
been adjusted to not limit user input per default.

This improves the corresponding rst to properly describe the changes made.

Resolves: #92825
Relates: #92659
Releases: master
Change-Id: I61b1ee1fed95251d1a3be4dc9e3c996fb033aa04
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/66611


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Josef Glatz <josefglatz@gmail.com>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Josef Glatz <josefglatz@gmail.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>