Update zuul doc to cover SIGUSR1 behavior.

Change-Id: I6dcd87a59d836dc22b67a2bee7c764e0b603a809
Reviewed-on: https://review.openstack.org/21543
Approved: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins