The wording here is a confusing to me. Maybe:

"A `Publisher` can reject calls to its `subscribe` method if it is unable or unwilling to serve them (e.g. because it is overwhelmed or bounded by a finite number of underlying resources, etc...). It does this by emitting `onError` into the `Subscriber` passed to `Publisher.subscribe`."

I'm not sure it's necessary to point out that a rejection involves *not* emitting `onSubscribe`. That should be implied.