Improve -X error message.

Modules/main.c

@@ -390,7 +390,7 @@ Py_Main(int argc, char **argv)
 			skipfirstline = 1;
 			break;
 
-		/* case 'X': reserved for non-standard arguments */
+		/* case 'X': reserved for implementation-specific arguments */
 
 		case 'U':
 			Py_UnicodeFlag++;

Python/getopt.c

@@ -88,7 +88,7 @@ int _PyOS_GetOpt(int argc, char **argv, char *optstring)
 
 	if (option == 'X') {
 		fprintf(stderr,
-			"-X is reserved for non-standard arguments\n");
+		  "-X is reserved for implementation-specific arguments\n");
 		return '_';
 	}