Tidy finds WebIDLs with no spec.

Closes #6689

components/script/dom/webidls/ServoHTMLParser.webidl

@@ -3,6 +3,9 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
+// This interface is entirely internal to Servo, and should not be accessible to
+// web pages.
+
 // FIXME: find a better way to hide this from content (#3688)
 [NoInterfaceObject]
 interface ServoHTMLParser {