add comment (see ITS#3652)

servers/slapd/backover.c

@@ -273,6 +273,7 @@ over_access_allowed(
 
 	for ( ; on; on = on->on_next ) {
 		if ( on->on_bi.bi_access_allowed ) {
+			/* NOTE: do not copy the structure until required */
 		 	if ( !SLAP_ISOVERLAY( op->o_bd ) ) {
  				db = *op->o_bd;
 				db.be_flags |= SLAP_DBFLAG_OVERLAY;
@@ -287,6 +288,11 @@ over_access_allowed(
 	}
 
 	if ( rc == SLAP_CB_CONTINUE && oi->oi_orig->bi_access_allowed ) {
+		/* NOTE: do not copy the structure until requiredy */
+		/* NOTE: by default, oi->oi_orig->bi_access_allowed == NULL;
+		 * only backends that implement a specific hook
+		 * should store it there; by default, slap_access_allowed()
+		 * is invoked if oi->oi_orig->bi_access_allowed == NULL */
 		if ( !SLAP_ISOVERLAY( op->o_bd ) ) {
  			db = *op->o_bd;
 			db.be_flags |= SLAP_DBFLAG_OVERLAY;