Add comment to StyleSheetIterator

Change-Id: I06db79b9c497d9dac4c4c8cff1f3ed3b36f0023f
Reviewed-on: https://gerrit.libreoffice.org/8487
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>