Document zuul-changes.py tool script.

* doc/source/zuul.rst: Mention zuul-changes.py and provide sample
syntax in the restarting section.

Change-Id: I05888ca40e16681111e56b9ebf92acc3dc36175b
Reviewed-on: https://review.openstack.org/23938
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins