docs: data messages documentation

proto/src/message/backend.rs

@@ -5,6 +5,8 @@ use crate::message::primitive::pgstring::PgString;
 use crate::message::proto_message::ProtoMessage;
 use bincode::{Decode, Encode};
 
+/// Backend messages sent from the server to the client.
+/// For more info visit the [`55.2.3. Message Formats`](https://www.postgresql.org/docs/current/protocol-message-formats.html)
 #[derive(Debug)]
 pub enum BackendMessage {
     AuthenticationOk(AuthenticationOkData),