spacemanager: fix forbidden property message

Patch https://rb.dcache.org/r/7168/ committed as 8ea811d576, marks
deprecated properties as forbidden.  Unfortunately the message for
some spacemanager properties was incorrect and following the advice
leaves sites broken.

This patch updates the comment.

Target: master
Requires-notes: no
Requires-book: no
Patch: https://rb.dcache.org/r/7309
Acked-by: Gerd Behrmann