response: add some notes about the errors

src/response.rs

@@ -4,10 +4,13 @@ pub type Response = Result<(), MpdError>;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize)]
 pub enum ErrorCode {
+    // NOTE: seemingly unused
     NotList = 1,
     Arg = 2,
+    // NOTE: only used for 'password' command
     Password = 3,
     Permission = 4,
+    // NOTE: Catchall error
     Unknown = 5,
 
     NoExist = 50,