[Extensions] Add feature documentation for id hashes

chrome/common/extensions/api/_manifest_features.json

Index: chrome/common/extensions/api/_manifest_features.json
diff --git a/chrome/common/extensions/api/_manifest_features.json b/chrome/common/extensions/api/_manifest_features.json
index 3b9d2f926f95c8c7bb4cd52eeb23b67fbfbc3805..ef7da84e9757593495f0b65b08e4998b71de9b72 100644
--- a/chrome/common/extensions/api/_manifest_features.json
+++ b/chrome/common/extensions/api/_manifest_features.json
@@ -3,17 +3,8 @@
 // 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": {