Fixed #21044 -- Documented django.core.urlresolvers.Resolver404

Thanks Keryn Knight for the suggestion.

docs/ref/exceptions.txt

@@ -131,6 +131,15 @@ URL Resolver exceptions
 
 URL Resolver exceptions are defined in :mod:`django.core.urlresolvers`.
 
+Resolver404
+--------------
+.. exception:: Resolver404
+
+    The :exc:`Resolver404` exception is raised by
+    :func:`django.core.urlresolvers.resolve()` if the path passed to
+    ``resolve()`` doesn't map to a view. It's a subclass of
+    :class:`django.http.Http404`
+
 NoReverseMatch
 --------------
 .. exception:: NoReverseMatch

docs/ref/urlresolvers.txt

@@ -102,8 +102,9 @@ worry about the ``urlconf`` parameter. The function returns a
 :class:`ResolverMatch` object that allows you
 to access various meta-data about the resolved URL.
 
-If the URL does not resolve, the function raises an
-:class:`~django.http.Http404` exception.
+If the URL does not resolve, the function raises a
+:exc:`~django.core.urlresolvers.Resolver404` exception (a subclass of
+:class:`~django.http.Http404`) .
 
 .. class:: ResolverMatch