[TASK] Clarify reset in page renderer comment

Also add tests to enforce this behavior.

Resolves: #58581
Releases: 6.2
Change-Id: Ife3894fe6dd5fc476bba9c9c74ae26bf310b9245
Reviewed-on: https://review.typo3.org/29582
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters