Document start day of week

Unlike cron apscheduler which is used by zuul uses Monday instead of
Sunday as start of week. Document this to make this clear.

Change-Id: I085c909ea4e15e7f93b3ceeefed720678a85d51d
diff --git a/doc/source/admin/drivers/timer.rst b/doc/source/admin/drivers/timer.rst
index c70df5c..c8afdd7 100644
--- a/doc/source/admin/drivers/timer.rst
+++ b/doc/source/admin/drivers/timer.rst
@@ -21,4 +21,4 @@
 **time**
   The time specification in cron syntax.  Only the 5 part syntax is
   supported, not the symbolic names.  Example: ``0 0 * * *`` runs at
-  midnight.
+  midnight. The first weekday is Monday.