[Extensions] Add feature documentation for id hashes

extensions/common/api/_behavior_features.json

Index: extensions/common/api/_behavior_features.json
diff --git a/extensions/common/api/_behavior_features.json b/extensions/common/api/_behavior_features.json
index c3177fa945c8cfa05455f7d3867ed2cf1438c92b..07fae0ebc5134dc04045a36138f371ef28311d01 100644
--- a/extensions/common/api/_behavior_features.json
+++ b/extensions/common/api/_behavior_features.json
@@ -4,18 +4,8 @@
 
 // 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": {