Documenting consensus messages

[slowli]

It was determined in #46 that consensus messages (e.g., Propose) are not sufficiently documented for now. Each such message could be commented like

// Request connected peers from the node `to`.
//
// ### Processing
//   * The message is authenticated by the pubkey `from`. 
//     It must be in the receiver's full node list
//   * If the message is properly authorized, the node responds with...
//
// ### Generation
// A node generates `RequestPeers` under such and such conditions...
message! {
     RequestPeers {

Note that consensus messages are slightly different from transaction messages defined by services; neither Processing, nor Generation sections can be straightforwardly translated for transaction messages (although these messages should probably be documented too). This is because tx message processing is encapsulated in the execute() method of the transaction (i.e., can be documented there); and there are no specific rules as to when ordinary tx messages are generated.

Proposed solution: I think some documentation for consensus messages is needed both here and in general Exonum docs. Message descriptions here could be useful in order to verify that messages are processed and generated as intended without needing to consult an external source. And they can be copy-pasted to the general docs if necessary.

[stanislav-tkach]

I would like to take this issue, although I will probably need some help in order to document things properly. Still, it would be a nice opportunity to understand consensus better.

[dj8yfo]

i assume, getting the doc actually generated depends on #17.

[dj8yfo]

@defuz the comments for macro-generated messages won't actually get to rustdoc, unless #71 handles it.
So, there should be /// -> #[doc="..."] pr.
merging fast with 1 approve. #92

[stanislav-tkach]

Technically #92 has been approved by @slowli, but that approval was dismissed by github after minor changes.

Additionally, the pull request was here for 7 days.