Re-phrase FIXME for toasting type 'm' columns for AO tables

Co-authored-by: NJesse Zhang <sbjesse@gmail.com>

src/backend/access/heap/tuptoaster.c

@@ -985,8 +985,11 @@ toast_insert_or_update_generic(Relation rel, GenericTuple newtup, GenericTuple o
 	 * externally unless really necessary.
 	 */
 	/*
-	 * GPDB_90_MERGE_FIXME: Should we do something like this with memtuples on
-	 * AO tables too?
+	 * FIXME: Should we do something like this with memtuples on
+	 * AO tables too? Currently we do not increase the target tuple size for AO
+	 * table, so there are occasions when columns of type 'm' will be stored
+	 * out-of-line but they could otherwise be accommodated in-block
+	 * c.f. upstream Postgres commit ca7c8168de76459380577eda56a3ed09b4f6195c
 	 */
 	if (!ismemtuple)
 		maxDataLen = TOAST_TUPLE_TARGET_MAIN - hoff;