Update mailing list documentation

docs/index.rst

@@ -1289,6 +1289,9 @@ Rather than repeat his fine work, here's a summary:
 
     List-Id: Example Mailing List <list.example.com>
 
+Modern mail clients and webmail services generate unsubscribe buttons based on
+these headers.
+
 **Do not**
 
 * Remove or modify any existing headers from the original message, including
@@ -1301,6 +1304,10 @@ and Reply All actions work like they would with any email message. Reply
 replies to the message sender, and Reply All replies to the sender and the
 list.
 
+Even without a subject prefix or body footer, mailing list users can still
+tell that a message came from the mailing list, because the message was sent
+to the mailing list post address, and not their email address.
+
 Configuration steps for common mailing list platforms are listed below.
 
 Mailman 2