Adding Channel to documentation

[hkateu]

Hi,
I added documentation for Channel based on issue2866.

I also think that for new users to easily follow the documentation, the concurrency primitives, we can do the following:

• Change the order in the docs to: Signal , Channel, Topic then Queue. Going from a Channel to a Topic seems logical since its a many-to-one vs many to many pub-sub relationship.
• Show that Topics are used for a many-to-many pub-sub scenario, since it's not explicitly mentioned in the docs.

Any feedback is welcome.