Docs for Channel

The documentation for Channel is currently no more than intellisense can tell me (except which framework versions it applies to...), which is not enough for a documentation page: https://docs.microsoft.com/en-us/dotnet/api/system.threading.channels.channel-2

Channel and Channel aren't that important here, since Channel is just for constructing channels and Channel is just Channel.

The big one should have at least some description and a simple example. I haven't fully understood the whole blog post yet, but things like

"A channel is basically a data structure that’s used to store produced data for a consumer to retrieve, and an appropriate synchronization to enable that to happen safely, while also enabling appropriate notifications in both directions."

would be a good start for a description. I will update this issue when understanding more of the blog post and getting a better idea about what to put in the docs.