Document TransformStream cleanup using "transformer.cleanup"

[lucacasonato]

The WHATWG Streams API was recently updated with a new hook for TransformStream cleanup. This allows cleaning up resources backing a transform stream when the stream is errored or aborted. Previously this sort of cleanup was only possible via the flush hook, which is only called when the readable stream naturally closes.

The spec PR can be found here: whatwg/streams#1283

[hamishwillee]

Thanks @lucacasonato . MDN documents things when there is a testable implementation on any of the major browser engines.

If not done by someone else, this will be picked up when https://bugzilla.mozilla.org/show_bug.cgi?id=1856103 merges (I added the dev-doc-needed keyword).