improve code docs in api2

Note: API methos should be declared pub, so that they show up in the generated docu.

src/api2/backup.rs

@@ -1,3 +1,5 @@
+//! Backup protocol (HTTP2 upgrade)
+
 use anyhow::{bail, format_err, Error};
 use futures::*;
 use hyper::header::{HeaderValue, UPGRADE};