Answers 204 No Content with no body: from then on nothing more about that segment is
delivered to the list. The segment stays saved,
and other lists that follow it keep doing so. Once no list of the organization follows it,
its unit of the watched-segments allowance is freed at once.
A list that does not follow the segment, and a list or segment that is not your
organization's, are 404 not_found, so a second DELETE is a 404, not a 204. Treat
that 404 as done when you retry. Following the segment again later takes a new baseline:
what entered it in between is not delivered.
Deleting the saved segment or the watchlist stops the follow as well. Not counted as a call.