Doc: ref links for "approval" requirement

The pipeline.require.approval logic is referenced at two other places
but lacks a link.  Add a label to easily browse to the approval
description.

Change-Id: Iba2e5a1db4023cd61444b2b8de53ffddfc03b16f
diff --git a/doc/source/zuul.rst b/doc/source/zuul.rst
index 6cb5d59..efe0de2 100644
--- a/doc/source/zuul.rst
+++ b/doc/source/zuul.rst
@@ -462,7 +462,8 @@
     This may be used for any event.  It requires that a certain kind
     of approval be present for the current patchset of the change (the
     approval could be added by the event in question).  It follows the
-    same syntax as the "approval" pipeline requirement below.
+    same syntax as the :ref:`"approval" pipeline requirement below
+    <pipeline-require-approval>`.
 
   **timer**
     This trigger will run based on a cron-style time specification.
@@ -497,7 +498,8 @@
     This may be used for any event.  It requires that a certain kind
     of approval be present for the current patchset of the change (the
     approval could be added by the event in question).  It follows the
-    same syntax as the "approval" pipeline requirement below.
+    same syntax as the :ref:`"approval" pipeline requirement below
+    <pipeline-require-approval>`.
 
 
 **require**
@@ -507,6 +509,8 @@
   the conditions specified here must be met or the item will not be
   enqueued.
 
+.. _pipeline-require-approval:
+
   **approval**
   This requires that a certain kind of approval be present for the
   current patchset of the change (the approval could be added by the