[Extensions] Add feature documentation for id hashes

extensions/common/api/_behavior_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 switches used to control Extension behaviour,
 // typically whitelist configuration.
-//
-// 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.
 
 {
   "whitelisted_for_incognito": {
     "channel": "stable",
     "extension_types": "all",
     // This is "external_component" for legacy reasons; it should be
     // unnecessary given there's a whitelist.
     "location": "external_component",
     "whitelist": [
       "D5736E4B5CF695CB93A2FB57E4FDC6E5AFAB6FE2",  // http://crbug.com/312900
@@ skipping 17 matching lines @@
     ]
   },
   "zoom_without_bubble": {
     "channel": "stable",
     "extension_types": "all",
     "whitelist": [
       "CBCC42ABED43A4B58FE3810E62AFFA010EB0349F"   // https://crbug.com/538252
     ]
   }
 }