[BUGFIX] Add advice to RTEmagic wizard

The update wizard that migrates RTE magic images relies on
sys_refindex for finding the magic images. There should be
a warning in the wizard description that users should be sure
their reference index is up to date.

Resolves: #49727
Releases: 6.1, 6.2
Change-Id: I7301a4a52174424655a76b58c7b7856f5f5209a2
Reviewed-on: https://review.typo3.org/22007
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters