[FOLLOWUP] Document how to use extensions as Composer root for testing

Also polish the language of the related ReST file a bit.

Relates: #94996
Releases: master
Change-Id: Id5cc50bcb518db37323bb15602fb56b9ae699861
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71024


Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Tested-by: Helmut Hummel <typo3@helhum.io>
Reviewed-by: Benni Mack <benni@typo3.org>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Helmut Hummel <typo3@helhum.io>