Add comment about intentional fallthrough in switch.

Coverity complained about an apparent missing "break" in a switch
added by bb140506df605fab.  The human-readable comments are pretty
clear that this is intentional, but add a standard /* FALL THRU */
comment to make it clear to tools too.

src/backend/utils/adt/tsginidx.c

@@ -240,6 +240,7 @@ TS_execute_ternary(GinChkVal *gcv, QueryItem *curitem)
 			 * treat OP_PHRASE as OP_AND with recheck requirement
 			 */
 			*gcv->need_recheck = true;
+			/* FALL THRU */
 
 		case OP_AND:
 			val1 = TS_execute_ternary(gcv, curitem + curitem->qoperator.left);