Clarify that datachannel close is reflected remotely

[jan-ivar]

WPT tests confirm this, but the spec is not super clear. The datachannel closing procedure:

It's unclear what "When that happens" means: teardown or algorithm being called? Normally we try to separate:

1. Normative requirement: When X happens, the UA MUST run this algorithm
2. Normative algorithm: To do X, run this algorithm

Evidence suggests it's 1.: It's called solely from dc.close(), yet prose suggests otherwise: "If the closing procedure has not started yet" and "Unless the procedure was initiated by channel.close,"

[jan-ivar]

AFAIK this has been part of datachannels since day one (even predating the "closing" event), so I suspect this is just poorly written. Marking editorial to clarify

[jan-ivar]

RFC 8831 supports this, saying "Closing of a data channel MUST be signaled".