[1.10.x] Documented url()'s 'regex' parameter.

Backport of 0a9bd266ff8e042f53e25928527a29ce896b4955 from master

docs/ref/urls.txt

@@ -33,6 +33,13 @@ Helper function to return a URL pattern for serving files in debug mode::
         ...
     ]
 
+The ``regex`` parameter should be a string or
+:func:`~django.utils.translation.ugettext_lazy()` (see
+:ref:`translating-urlpatterns`) that contains a regular expression compatible
+with Python's :py:mod:`re` module. Strings typically use raw string syntax
+(``r''``) so that they can contain sequences like ``\d`` without the need to
+escape the backslash with another backslash.
+
 The ``view`` parameter is a view function or the result of
 :meth:`~django.views.generic.base.View.as_view` for class-based views. It can
 also be an :func:`include`.

docs/topics/i18n/translation.txt

@@ -1557,6 +1557,7 @@ will be::
     Ensure that you don't have non-prefixed URL patterns that might collide
     with an automatically-added language prefix.
 
+.. _translating-urlpatterns:
 
 Translating URL patterns
 ------------------------