[TASK] Improve BE description of mount point overlay

Give a more precise description of the mount point overlay behaviour:

- Update labels of the page tree.
- Update lables of the page form.

Releases: master, 8.7
Resolves: #81163
Change-Id: Ib1046e5caf29e7126b05446d4d3063bb87e3c9fa
Reviewed-on: https://review.typo3.org/52736


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Henrik Elsner <helsner@dfau.de>
Tested-by: Henrik Elsner <helsner@dfau.de>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>