Improve Protocol Documentation.

First of all, thanks for the documentation. I appreciate the effort that goes into it. Thing is, I just read the protocol section in the documentation here and I have to say it leaves me with more questions than answers.

What I got from it is basically that protocol allows nodes to communicate in a particular way. But I know that already from it being called protocol and I do not have to read the piece. The information about protocol Ids and protocol negotiation also does not provide much value.

I think what I am missing is:

• What is a structure of a protocol in libp2p? What interface does it provide, what are the rules to follow etc
• Do not just list examples of inbuilt protocol (Ping, Identify, secio etc) but show how to use them and relate it to the interface they provide.
• And maybe an advance section that quickly shows how to write a custom protocol.

I think the documentation on protocol will be improved is these points are taken into consideration.