Clarify docs for optional template pipelines

The documentation could be read to imply a check, gate, et cetera
pipeline must contain a job, when in fact pipeline definition is optional
in templates.
.
Inform the reader of the documentation as it isn't entirely clear for
folks maintaing projects where the developer is not a zuul developer.

Change-Id: If386e9431d486f8cea2ebde52c555305e1278c7d