[Extensions] Add feature documentation for id hashes

chrome/common/extensions/api/_manifest_features.json

 // Copyright (c) 2012 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 manifest keys implemented under src/chrome.
-// See extensions/common/features/* to understand this file, in particular
-// feature.h, simple_feature.h, and base_feature_provider.h.
-
-// 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.
+// See chrome/common/extensions/api/_features.md to understand this file, as
+// well as feature.h, simple_feature.h, and base_feature_provider.h.
 
 {
   "about_page": {
     "channel": "stable",
     "extension_types": ["shared_module"]
   },
   "app.icon_color": {
     "channel": "stable",
     "extension_types": ["hosted_app"]
   },
@@ skipping 239 matching lines @@
   },
   "update_url": {
     "channel": "stable",
     "extension_types": "all"
   },
   "url_handlers": {
     "channel": "stable",
     "extension_types": ["platform_app"]
   }
 }