Document how to reply to channel requests

[qsantos]

See #156.

Using Rust typing system would be less error-prone. Maybe the pty-req case in server_read_authenticated could handle it in two ways:

• call Handle::channel_success or Handle::channel_failure depending on the value returned by the handler;
• always call Handle::channel_successafter calling the handler, assuming that, if the user callsHandle::channel_failure` in the handler, it will have priority; feels very hacky though.

[qsantos]

Regarding the second part of my comment, I just realized that this is what #348 tries to do.

[Eugeny]

Thanks! @all-contributors add @qsantos for docs

[allcontributors]

@Eugeny

I've put up a pull request to add @qsantos! 🎉