Document Cross-project pipeline triggers `pipelines` filter

Cross-project pipeline triggers don't seem to be respected as triggers. When your downstream pipeline has only and except, for triggers, I'd expect that to be respected, but it seems to be ignored. Furthermore, the pipeline list gives no indication that the pipeline was triggered. Previously, with old triggers API, it would say by API. Perhaps because the new trigger mechanism impersonates the person who triggered the upstream pipeline, the downstream pipeline doesn't know it was a trigger at all.

So either we need to respect triggers for only and except, or we add a new filter for cross-project pipelines. But likely we need to make the former work for any regular pipeline triggers using the new API, although I haven't tested that hypothesis.

/cc @bikebilly @ayufan