[TASK] Document options.pageTree.altElementBrowserMountPointsAppend

Please **do not merge** before https://review.typo3.org/#/c/31898/ is submitted.

Thank you :)

Documents the option introduced in #50780 that allows appending
the alternative element browser mount points.

Resolves: #63009
Releases: master
Change-Id: Idbfcf6a21419716df619adb51342ac0e656c387c