[Extensions] Add feature documentation for id hashes

extensions/common/api/_api_features.json

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // This features file defines extension APIs implemented under src/extensions.
-// See extensions/common/features/* to understand this file, in particular
-// feature.h, simple_feature.h, and base_feature_provider.h.
+// See chrome/common/extensions/api/_features.md to understand this file, as
+// well as feature.h, simple_feature.h, and base_feature_provider.h.
 //
 // Note that specifying "web_page", "blessed_web_page", or "all" as a context
 // type will require manually updating chrome/renderer/resources/dispatcher.cc.
 
-// To add a new whitelisted ID, SHA-1 it and force it to uppercase. In Bash:
-//
-// $ echo -n "aaaabbbbccccddddeeeeffffgggghhhh" | \
-// sha1sum | tr '[:lower:]' '[:upper:]'
-// 9A0417016F345C934A1A88F55CA17C05014EEEBA  -
-//
-// Google employees: please update http://go/chrome-api-whitelist to map
-// hashes back to ids.
-
 {
   "alarms": {
     "dependencies": ["permission:alarms"],
     "contexts": ["blessed_extension"]
   },
   "app.runtime": [{
     "channel": "stable",
     "contexts": ["blessed_extension"],
     "extension_types": ["platform_app"],
     "noparent": true
@@ skipping 474 matching lines @@
   }, {
     "channel": "stable",
     "contexts": ["webui"],
     "matches": [
       "chrome://chrome-signin/*",
       "chrome://media-router/*",
       "chrome://oobe/*"
     ]
   }]
 }