[BUGFIX] Description of reference_uid property

Improve the description of the reference_uid property,
hopefully to avoid users confusing field names in the TYPO3 DB
and in the external data.

Resolves: #60808
Releases: 2.3
Change-Id: Iaed3b31eaf6899a4ac002390d05ef63d92765d22
Reviewed-on: http://review.typo3.org/32026
Reviewed-by: Francois Suter <francois@typo3.org>
Tested-by: Francois Suter <francois@typo3.org>