Add docs for Channel

[j8r]

Fixes #5801

I'm not very familiar with channels and its concepts, this docs can be improved.

[ysbaddaden]

Can you add more actual documentation? For example what's a channel? What are they used for? What safety do they entail? What are Buffered and Unbuffered channels? What's their difference and strong/weak points? When will operations block —because even a buffered channel can block.

[j8r]

@ysbaddaden I wish I could but I have not the experience to go into the details, and tell pros and cons of buffered vs unbuffered channels. Mine barely reach Channel(T).new, spawn, send and receive 😅
That's a reason why I opened this PR, I think it's important to do our best to document this major language feature.

[sdogruyol]

I think, this is good enough for the start. @j8r do you plan to improve this in the future?

[j8r]

@sdogruyol Maybe, I don't know when I would have enough experience/knowledge with Channels to further improve the documentation. I won't probably the best one for this task.