Clarify catch-all aliases with alias-domains

The docs don't yet mention alias domains and their effect when using catch-all aliases for a domain within mailcow. This added example should clarify that any mail send to an alias domain will in fact be delivered to the catch-all alias on the target domain, which is in contrast to specific aliases added for a single mailbox.

docs/model-sender_rcv.md

@@ -21,6 +21,14 @@ It is important to know, that you are not able to receive mail for `my-alias@my-
 
     me@example.org is NOT known as alias@alias.com.
 
+Please note that this does not apply to catch-all aliases:
+
+    Alias domain alias.com is added and assigned to primary domain example.org
+    me@example.org is assigned the catch-all alias @example.org
+    me@example.org is still just known as me@example.org, which is the only available send-as option
+    
+    Any email send to alias.com will match the catch-all alias for example.org
+
 Administrators and domain administrators can edit mailboxes to allow specific users to send as other mailbox users ("delegate" them).
 
 You can choose between mailbox users or completely disable the sender check for domains.