Add zuul.timeout variable to jobs

The existing devstack-gate system uses this.

Change-Id: I7e324397dfe22ec11c157dda13d7f59de40211f7
diff --git a/doc/source/user/jobs.rst b/doc/source/user/jobs.rst
index 4c8b784..837fb17 100644
--- a/doc/source/user/jobs.rst
+++ b/doc/source/user/jobs.rst
@@ -248,6 +248,10 @@
 
       The name of the current Zuul tenant.
 
+   .. var:: timeout
+
+      The job timeout, in seconds.
+
    .. var:: jobtags
 
       A list of tags associated with the job.  Not to be confused with