diff --git a/railties/test/isolation/abstract_unit.rb b/railties/test/isolation/abstract_unit.rb
index <HASH>..<HASH> 100644
--- a/railties/test/isolation/abstract_unit.rb
+++ b/railties/test/isolation/abstract_unit.rb
@@ -90,6 +90,7 @@ module TestHelpers
   end
 
   module Generation
+    # Build an application by invoking the generator and going through the whole stack.
     def build_app(options = {})
       FileUtils.rm_rf(app_path)
       FileUtils.cp_r(tmp_path('app_template'), app_path)
@@ -115,6 +116,8 @@ module TestHelpers
       add_to_config 'config.secret_token = "3b7cd727ee24e8444053437c36cc66c4"; config.session_store :cookie_store, :key => "_myapp_session"; config.active_support.deprecation = :log'
     end
 
+    # Make a very basic app, without creating the whole directory structure.
+    # This is faster and simpler than the method above.
     def make_basic_app
       require "rails"
       require "action_controller/railtie"
Add some very simple docs.