Update the comments for the new functionality

lib/gitlab/cache/request_store_wrap.rb

@@ -2,7 +2,8 @@ module Gitlab
   module Cache
     # This module provides a simple way to cache values in RequestStore,
     # and the cache key would be based on the class name, method name,
-    # customized instance level values, and arguments.
+    # optionally customized instance level values, optionally customized
+    # method level values, and optional method arguments.
     #
     # A simple example:
     #
@@ -10,7 +11,7 @@ module Gitlab
     #   extend Gitlab::Cache::RequestStoreWrap
     #
     #   request_store_wrap_key do
-    #     [user.id, project.id]
+    #     [user&.id, project&.id]
     #   end
     #
     #   request_store_wrap def can_push_to_branch?(ref)
@@ -19,7 +20,22 @@ module Gitlab
     # end
     #
     # This way, the result of `can_push_to_branch?` would be cached in
-    # `RequestStore.store` based on the cache key.
+    # `RequestStore.store` based on the cache key. If RequestStore is not
+    # currently active, then it would be stored in a hash saved in an
+    # instance variable, so the cache logic would be the same.
+    # Here's another example using customized method level values:
+    #
+    # class Commit
+    #   extend Gitlab::Cache::RequestStoreWrap
+    #
+    #   def author
+    #     User.find_by_any_email(author_email.downcase)
+    #   end
+    #   request_store_wrap(:author) { author_email.downcase }
+    # end
+    #
+    # So that we could have different strategies for different methods
+    #
     module RequestStoreWrap
       def self.extended(klass)
         return if klass < self