[Extensions] Add feature documentation for id hashes

extensions/common/api/_manifest_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 manifest keys implemented under src/extensions.
-// 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.
 
 {
   "app": {
     "channel": "stable",
     "extension_types": ["legacy_packaged_app", "hosted_app", "platform_app"]
   },
   // The default platform app CSP can only be overridden by whitelisted apps.
   // This is a separate key from the top-level content_security_policy one since
   // we can't combine type restrictions with whitelisted ID restrictions. If
   // there is a need for additional whitelisted entries, the feature system
@@ skipping 298 matching lines @@
       "extension_types": ["platform_app"],
       "location": "component"
     }
   ],
   "webview": {
     "channel": "stable",
     "extension_types": ["platform_app"],
     "min_manifest_version": 2
   }
 }