Document the existence of zuul_success job var

Depends-On: I96af116b9a830e5d67a9b4fd7fa7451620ad3b16

Change-Id: I98b31f12bfb86dc12adc420352bc01eef43f3627
Signed-off-by: Jesse Keating <omgjlk@us.ibm.com>
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 51a148a..3d24f5d 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -314,6 +314,13 @@
          The patchset identifier for the change.  If a change is
          revised, this will have a different value.
 
+.. var:: zuul_success
+
+   Post run playbook(s) will be passed this variable to indicate if the run
+   phase of the job was successful or not. This variable is meant to be used
+   with the `boolean` filter.
+
+
 Change Items
 ++++++++++++