Make it clearer what we mean by 'enabling' an html sidebar; fixes #98

Author: bitprophet

docs/customization.rst

@@ -134,8 +134,8 @@ Variables and feature toggles
 
 * ``extra_nav_links``: Dictionary mapping link names to link targets; these
   will be added in a UL below the main sidebar navigation (provided you've
-  enabled ``navigation.html``.) Useful for static links outside your Sphinx
-  doctree.
+  enabled ``navigation.html`` via the ``html_sidebars`` option; see
+  :doc:`installation`.) Useful for static links outside your Sphinx doctree.
 * ``sidebar_includehidden``: Boolean determining whether the TOC sidebar
   should include hidden Sphinx toctree elements. Defaults to ``true`` so you
   can use ``:hidden:`` in your index page's root toctree & avoid having 2x