[1.6.x] Updated links in admin docs to use :ref:

per Sphinx docs: "Using ref is advised over standard reStructuredText
links to sections because it works across files, when section headings
are changed, and for all builders that support cross-references.

Backport of 13409b932a from master

docs/ref/contrib/admin/index.txt

@@ -343,7 +343,7 @@ subclass::
     form rather than specifying an entirely new one by using the
     :meth:`ModelAdmin.get_form` method.
 
-    For an example see the section `Adding custom validation to the admin`_.
+    For an example see the section :ref:`admin-custom-validation`.
 
     .. admonition:: Note
 
@@ -1067,7 +1067,7 @@ subclass::
 Custom template options
 ~~~~~~~~~~~~~~~~~~~~~~~
 
-The `Overriding Admin Templates`_ section describes how to override or extend
+The :ref:`admin-overriding-templates` section describes how to override or extend
 the default admin templates.  Use the following options to override the default
 templates used by the :class:`ModelAdmin` views:
 
@@ -1625,6 +1625,8 @@ return the uncompressed versions of the various JavaScript files, including
 
 .. _jQuery: http://jquery.com
 
+.. _admin-custom-validation:
+
 Adding custom validation to the admin
 -------------------------------------
 
@@ -2042,6 +2044,8 @@ other inline. In your ``admin.py`` for this example app::
 See the :doc:`contenttypes documentation </ref/contrib/contenttypes>` for more
 specific information.
 
+.. _admin-overriding-templates:
+
 Overriding admin templates
 ==========================
 
@@ -2173,7 +2177,7 @@ creating your own ``AdminSite`` instance (see below), and changing the
 ------------------------
 
 Templates can override or extend base admin templates as described in
-`Overriding Admin Templates`_.
+:ref:`admin-overriding-templates`.
 
 .. attribute:: AdminSite.index_template