Docs improvements - clarification of channel closure

[womensrights]

There is limited context in the existing documentation on channel closures.

It would be good to provide more context on channel closures, particularly for unordered channels as questions are often asked on this topic:

• clarify in practise what is meant by the ACTOR (referred to in the specification) that can close the channel
• why this is not something that happens in normal circumstances
• what are the circumstances where an unordered channel closure would happen