Add hints for html_sidebars

sphinx/templates/quickstart/conf.py_t

@@ -110,6 +110,11 @@ html_static_path = ['{{ dot }}static']
 # Custom sidebar templates, must be a dictionary that maps document names
 # to template names.
 #
+# The default sidebars (for documents that don't match any pattern) are
+# defined by theme itself.  Builtin themes are using these templates by
+# default: ``['localtoc.html', 'relations.html', 'sourcelink.html',
+# 'searchbox.html']``.
+#
 # html_sidebars = {}