Document list of configuration items for include/exclude

Also change the word to Configuration Item since that's what it's called
in config.rst and in error messages. Leave TenantParser use of classes.

Change-Id: I1844665a4f6dd63d517cab68049fae6ed7fcca68
diff --git a/doc/source/user/config.rst b/doc/source/user/config.rst
index e356d44..cad2167 100644
--- a/doc/source/user/config.rst
+++ b/doc/source/user/config.rst
@@ -57,6 +57,8 @@
 any Zuul-managed repository, the new configuration takes effect
 immediately.
 
+.. _configuration-items:
+
 Configuration Items
 -------------------