Update descriptions for templates
Update descriptions for templates

Update some descriptions, add cross-references.

Add missing descriptions, now all templates in
zuul.d/project-templates.yaml have descriptions.

Change-Id: I1a51709985edd1b34b8cc28df29fda6727fc246c

openstack-infra/openstack-zuul-jobs,openstack-infra/openstack-zuul-jobs