Clarified 404.html usage, excplicitly stated that it's used when DEBUG is False

Thanks to Keryn Knight, Curtis Maloney and Tim Graham for their reviews.

docs/topics/http/views.txt

@@ -122,14 +122,15 @@ Example usage::
             raise Http404("Poll does not exist")
         return render_to_response('polls/detail.html', {'poll': p})
 
-In order to use the ``Http404`` exception to its fullest, you should create a
-template that is displayed when a 404 error is raised. This template should be
-called ``404.html`` and located in the top level of your template tree.
-
-If you provide a message when raising an ``Http404`` exception, it will appear
-in the standard 404 template displayed when :setting:`DEBUG` is ``True``. Use
-these messages for debugging purposes; they generally aren't suitable for use
-in a production 404 template.
+In order to show customized HTML when Django returns a 404, you can create an
+HTML template named ``404.html`` and place it in the top level of your
+template tree. This template will then be served when :setting:`DEBUG` is set
+to ``False``.
+
+When :setting:`DEBUG` is ``True``, you can provide a message to ``Http404`` and
+it will appear in the standard 404 debug template. Use these messages for
+debugging purposes; they generally aren't suitable for use in a production 404
+template.
 
 .. _customizing-error-views: