Clarify comment about temporal foreign keys

In RI_ConstraintInfo, period_contained_by_oper and
period_intersect_oper can take either anyrange or anymultirange.

Author: Paul A. Jungwirth <pj@illuminatedcomputing.com>
Discussion: https://www.postgresql.org/message-id/CA%2BrenyWzDth%2BjqLZA2L2Cezs3wE%2BWX-5P8W2EOVx_zfFD%3Daicg%40mail.gmail.com

src/backend/utils/adt/ri_triggers.c

@@ -126,9 +126,11 @@ typedef struct RI_ConstraintInfo
 	Oid			pf_eq_oprs[RI_MAX_NUMKEYS]; /* equality operators (PK = FK) */
 	Oid			pp_eq_oprs[RI_MAX_NUMKEYS]; /* equality operators (PK = PK) */
 	Oid			ff_eq_oprs[RI_MAX_NUMKEYS]; /* equality operators (FK = FK) */
-	Oid			period_contained_by_oper;	/* anyrange <@ anyrange */
+	Oid			period_contained_by_oper;	/* anyrange <@ anyrange (or
+											 * multiranges) */
 	Oid			agged_period_contained_by_oper; /* fkattr <@ range_agg(pkattr) */
-	Oid			period_intersect_oper;	/* anyrange * anyrange */
+	Oid			period_intersect_oper;	/* anyrange * anyrange (or
+										 * multiranges) */
 	dlist_node	valid_link;		/* Link in list of valid entries */
 } RI_ConstraintInfo;