style: document intentional fallthrough

cal.c

@@ -197,7 +197,7 @@ main(int argc, char *argv[])
 	case 2:
 		month = estrtonum(argv[0], 1, 12);
 		argv++;
-	case 1:
+	case 1: /* fallthrough */
 		year = estrtonum(argv[0], 0, INT_MAX);
 		break;
 	case 0:

readlink.c

@@ -83,7 +83,7 @@ mdone:
 	case 'e':
 		if (stat(argv[0], &st) < 0)
 			eprintf("stat %s:", argv[0]);
-	case 'f':
+	case 'f': /* fallthrough */
 		if (!realpath(argv[0], b))
 			eprintf("realpath %s:", argv[0]);
 		break;