document isopen(::Channel) (#56376)
This PR has two purposes --
1) Add some documentation for public API
2) Add a small note about a footgun I've hit a few times: `!isopen(ch)`
does not mean that you are "done" with the channel because buffered
channels can still have items left in them that need to be taken.
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Co-authored-by: CY Han <cyhan.dev@outlook.com>
