[Extensions] Add feature documentation for id hashes

extensions/common/api/_manifest_features.json

Index: extensions/common/api/_manifest_features.json
diff --git a/extensions/common/api/_manifest_features.json b/extensions/common/api/_manifest_features.json
index 841f4b159ab54e55cacb94e37f34ebd87c25ce45..f9f238f602b8de14fef26b1a7fbb69d01c1c0158 100644
--- a/extensions/common/api/_manifest_features.json
+++ b/extensions/common/api/_manifest_features.json
@@ -3,17 +3,8 @@
 // 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": {