fixup! Firefox preference overrides.

Document why we are locking toolkit.telemetry.enabled.

browser/app/profile/001-base-profile.js

@@ -119,6 +119,7 @@ pref("datareporting.healthreport.uploadEnabled", false);
 pref("datareporting.policy.dataSubmissionEnabled", false);
 // Make sure Unified Telemetry is really disabled, see: #18738.
 pref("toolkit.telemetry.unified", false);
+// This needs to be locked, or nightly builds will automatically lock it to true
 pref("toolkit.telemetry.enabled", false, locked);
 pref("toolkit.telemetry.server", "data:,");
 pref("toolkit.telemetry.archive.enabled", false);