spec/p2p: Document the methods offered by the p2p layer to reactors

[cason]

Reactor is the generic name for a component that interacts with the p2p communication layer.

This issue aims to complement the documentation of the interaction between p2p layer and reactors introduced by #714, by focusing on the interface (methods) the p2p layer offers to a Reactor.

Once a Reactor registers with the p2p layer, it receives back from it a reference to the Switch, which is the main component of the p2p layer, with a vast interface. Some of the Switch methods can/should be used by reactors in order to control the operation of the p2p layer.

In addition, once a peer connects to the node, the p2p layer passes to all the registered reactors a Peer handle. This handler offers a number of methods with which the reactor can interact or configure the peer. The most important are the two sending methods, partially documented in the code and here.

Definition of done:

• The documentation of the methods is more detailed and updated
• The old documentation points to the new version of the documentation
• Part of the operation of the documented methods, in particular the methods offered by Peer, are modeled using Quint