[ci skip] Add `ActionDispatch::ActionableExceptions` to default middleware docs

guides/source/rails_on_rack.md

@@ -116,6 +116,7 @@ use Rails::Rack::Logger
 use ActionDispatch::ShowExceptions
 use WebConsole::Middleware
 use ActionDispatch::DebugExceptions
+use ActionDispatch::ActionableExceptions
 use ActionDispatch::Reloader
 use ActionDispatch::Callbacks
 use ActiveRecord::Migration::CheckPending
@@ -261,6 +262,10 @@ Much of Action Controller's functionality is implemented as Middlewares. The fol
 
 * Responsible for logging exceptions and showing a debugging page in case the request is local.
 
+**`ActionDispatch::ActionableExceptions`**
+
+* Provides a way to dispatch actions from Rails' error pages.
+
 **`ActionDispatch::Reloader`**
 
 * Provides prepare and cleanup callbacks, intended to assist with code reloading during development.