Support cross-references based on zuul:attr

Add a new role: :zuul:attr: which will cross reference to a zuul:attr:
directive.

Change-Id: I69a65a9f4a0330f664f6183180872d459d234e72
diff --git a/doc/source/admin/drivers/gerrit.rst b/doc/source/admin/drivers/gerrit.rst
index 91a3510..bc203cb 100644
--- a/doc/source/admin/drivers/gerrit.rst
+++ b/doc/source/admin/drivers/gerrit.rst
@@ -192,7 +192,7 @@
 named *my-gerrit* must have a Code Review vote of +2 in order to be
 enqueued into the pipeline.
 
-.. zuul:attr:: pipeline.require.<source>
+.. zuul:attr:: pipeline.require.<gerrit source>
 
    The dictionary passed to the Gerrit pipeline `require` attribute
    supports the following attributes:
@@ -250,7 +250,7 @@
       A string value that corresponds with the status of the change
       reported by the trigger.
 
-.. zuul:attr:: pipeline.reject.<source>
+.. zuul:attr:: pipeline.reject.<gerrit source>
 
    The `reject` attribute is the mirror of the `require` attribute.  It
    also accepts a dictionary under the connection name.  This
diff --git a/doc/source/admin/drivers/github.rst b/doc/source/admin/drivers/github.rst
index c884be2..97492f6 100644
--- a/doc/source/admin/drivers/github.rst
+++ b/doc/source/admin/drivers/github.rst
@@ -216,7 +216,7 @@
 named *my-github* must have an approved code review in order to be
 enqueued into the pipeline.
 
-.. zuul:attr:: pipeline.require.<source>
+.. zuul:attr:: pipeline.require.<github source>
 
    The dictionary passed to the GitHub pipeline `require` attribute
    supports the following attributes:
@@ -290,7 +290,7 @@
       indicated label (or labels).
 
 
-.. zuul:attr:: pipeline.reject.<source>
+.. zuul:attr:: pipeline.reject.<github source>
 
    The `reject` attribute is the mirror of the `require` attribute.  It
    also accepts a dictionary under the connection name.  This