Document the meaning of empty Message.subject

src/chat.rs

@@ -3397,7 +3397,7 @@ pub async fn forward_msgs(context: &Context, msg_ids: &[MsgId], chat_id: ChatId)
             msg.param.remove(Param::WebxdcSummaryTimestamp);
             msg.in_reply_to = None;
 
-            // do not leak data as group names; a default subject is generated by mimfactory
+            // do not leak data as group names; a default subject is generated by mimefactory
             msg.subject = "".to_string();
 
             let new_msg_id: MsgId;

src/message.rs

@@ -265,6 +265,8 @@ pub struct Message {
     pub(crate) text: Option<String>,
 
     /// Message subject.
+    ///
+    /// If empty, a default subject will be generated when sending.
     pub(crate) subject: String,
 
     /// `Message-ID` header value.