Add comment for missing break in switch

For clarity, following other sites, and to silence Coverity.

src/backend/utils/adt/formatting.c

@@ -1104,6 +1104,7 @@ NUMDesc_prepare(NUMDesc *num, FormatNode *n)
 			case NUM_D:
 				num->flag |= NUM_F_LDECIMAL;
 				num->need_locale = TRUE;
+				/* FALLTHROUGH */
 			case NUM_DEC:
 				if (IS_DECIMAL(num))
 					ereport(ERROR,