Added some internal links to render_to_string documentation.

    positional argument.

context_instance

    An instance of ``Context`` or a subclass (e.g., an instance of

    ``RequestContext``) to use as the template's context. This can

    also be passed as the third positional argument.

    An instance of :class:`~django.template.Context` or a subclass (e.g., an

    instance of :class:`~django.template.RequestContext`) to use as the

    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`