Add ZUUL_VOTING parameter

So that jobs, but especially post-processing such as elastic-recheck,
can tell whether Zuul considers a job voting or not.

Change-Id: I40746bb77aab900c1dd2637f940c14f72a904a61
diff --git a/doc/source/launchers.rst b/doc/source/launchers.rst
index b95354f..0a1e0e7 100644
--- a/doc/source/launchers.rst
+++ b/doc/source/launchers.rst
@@ -66,6 +66,11 @@
 **LOG_PATH**
   zuul also suggests a unique path for logs to the worker. This is
   "BASE_LOG_PATH/pipeline-name/job-name/uuid"
+**ZUUL_VOTING**
+  Whether Zuul considers this job voting or not.  Note that if Zuul is
+  reconfigured during the run, the voting status of a job may change
+  and this value will be out of date.  Values are '1' if voting, '0'
+  otherwise.
 
 Change related parameters
 ~~~~~~~~~~~~~~~~~~~~~~~~~