f use link references so its clear what capacity means

Author: TheBlueMatt

lightning/src/ln/peer_channel_encryptor.rs

@@ -431,8 +431,11 @@ impl PeerChannelEncryptor {
 	///
 	/// `msgbuf` must begin with 16 + 2 dummy/0 bytes, which will be filled with the encrypted
 	/// message length and its MAC. It should then be followed by the message bytes themselves
-	/// (including the two byte message type), and have an extra 16 bytes of capacity remaining for
-	/// the message MAC, which will be appended to the vec.
+	/// (including the two byte message type).
+	///
+	/// For effeciency, the [`Vec::capacity`] should be at least 16 bytes larger than the
+	/// [`Vec::length`], to avoid reallocating for the message MAC, which will be appended to the
+	/// vec.
 	fn encrypt_message_with_header_0s(&mut self, msgbuf: &mut Vec<u8>) {
 		let msg_len = msgbuf.len() - 16 - 2;
 		if msg_len > LN_MAX_MSG_LEN {