[DOCS] Define reference index in ManageReferenceIndex.rst

Missing definition provided and wording corrected.

Releases: main, 12.4
Resolves: #104429
Change-Id: I59bca54826557e7633ce837c73df6d38cda24742
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/85294


Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Lina Wolf <112@linawolf.de>
Reviewed-by: Lina Wolf <112@linawolf.de>
Tested-by: core-ci <typo3@b13.com>