Add a note on ActionController guide about 404

[ci skip]

guides/source/action_controller_overview.md

@@ -1118,7 +1118,7 @@ Rails default exception handling displays a "500 Server Error" message for all e
 
 ### The Default 500 and 404 Templates
 
-By default a production application will render either a 404 or a 500 error message. These messages are contained in static HTML files in the `public` folder, in `404.html` and `500.html` respectively. You can customize these files to add some extra information and style, but remember that they are static HTML; i.e. you can't use ERB, SCSS, CoffeeScript, or layouts for them.
+By default a production application will render either a 404 or a 500 error message, in the development environment all unhandled exceptions are raised. These messages are contained in static HTML files in the `public` folder, in `404.html` and `500.html` respectively. You can customize these files to add some extra information and style, but remember that they are static HTML; i.e. you can't use ERB, SCSS, CoffeeScript, or layouts for them.
 
 ### `rescue_from`
 
@@ -1174,6 +1174,10 @@ end
 
 WARNING: You shouldn't do `rescue_from Exception` or `rescue_from StandardError` unless you have a particular reason as it will cause serious side-effects (e.g. you won't be able to see exception details and tracebacks during development).
 
+NOTE: When running in the production environment, all
+`ActiveRecord::RecordNotFound` errors render the 404 error page. Unless you need
+a custom behavior you don't need to handle this.
+
 NOTE: Certain exceptions are only rescuable from the `ApplicationController` class, as they are raised before the controller gets initialized and the action gets executed.
 
 Force HTTPS protocol