add documentation for Rails.env.local?

railties/lib/rails.rb

@@ -71,6 +71,7 @@ def root
     #   Rails.env # => "development"
     #   Rails.env.development? # => true
     #   Rails.env.production? # => false
+    #   Rails.env.local? # => true              true for "development" and "test", false for anything else
     def env
       @_env ||= ActiveSupport::EnvironmentInquirer.new(ENV["RAILS_ENV"].presence || ENV["RACK_ENV"].presence || "development")
     end