Update comment to say that Modified is for internal use only.

rustfmt-config/src/options.rs

@@ -176,7 +176,7 @@ configuration_option_enum! { WriteMode:
     Plain,
     // Outputs a checkstyle XML file.
     Checkstyle,
-    // Output the changed lines
+    // Output the changed lines (for internal value only)
     Modified,
 }