Added some internal links to render_to_string documentation.

docs/ref/templates/api.txt

@@ -835,9 +835,9 @@ dictionary
     positional argument.
 
 context_instance
-    An instance of ``Context`` or a subclass (e.g., an instance of
+    An instance of :class:`~django.template.Context` or a subclass (e.g., an
-    ``RequestContext``) to use as the template's context. This can
+    instance of :class:`~django.template.RequestContext`) to use as the
-    also be passed as the third positional argument.
+    template's context. This can also be passed as the third positional argument.
 
 See also the :func:`~django.shortcuts.render_to_response()` shortcut, which
 calls ``render_to_string`` and feeds the result into an :class:`~django.http.HttpResponse`