add some doc comments

src/lib.rs

@@ -299,7 +299,10 @@ impl SerializedMessage {
 /// Handshake between client and server (peers do not use).
 #[derive(Eq, Hash, PartialEq)]
 pub struct Handshake {
+    /// Who is sending this handshake.
     pub sender: String,
+    /// For normal messages, the group the message was sent to.
+    /// For receipts, the client the receipt is for.
     pub group: String,
 }
 
@@ -319,6 +322,7 @@ pub async fn get_handshake<T: AsyncReadExt + std::marker::Unpin>(
     Ok(Handshake { sender, group })
 }
 
+/// A reference to a Handshake's fields.
 pub struct HandshakeRef<'a> {
     pub sender: &'a str,
     pub group: &'a str,