added docs for ActionDispatch::Request::Session#create

actionpack/lib/action_dispatch/request/session.rb

@@ -9,7 +9,9 @@ class Session # :nodoc:
 
       # Singleton object used to determine if an optional param wasn't specified
       Unspecified = Object.new
-
+      
+      # Creates a session hash, merging the properties of the previous session if any
+      
       def self.create(store, env, default_options)
         session_was = find env
         session     = Request::Session.new(store, env)