updated documentation

lib/python/Products/MailHost/README.txt

@@ -7,7 +7,12 @@ MailHost
   Email can optionally be encoded using Base64, Quoted-Printable
   or UUEncode encoding.
 
-  MailHost provides integration with the Zope transaction system
-  and optional support for asynchronous mail delivery.
+  MailHost provides integration with the Zope transaction system and optional
+  support for asynchronous mail delivery. Asynchronous mail delivery is
+  implemented using a queue and a dedicated thread processing the queue. The
+  thread is (re)-started automatically when sending an email. The thread can be
+  startet manually (in case of restart) by calling its
+  manage_restartQueueThread?action=start method through HTTP. There is
+  currently no possibility to start the thread at Zope startup time.
 
   Supports TLS/SSL encryption (requires Python compiled with SSL support)