[Mesenger] Mention that some option doesn't have docs

Author: javiereguiluz

messenger.rst

@@ -1465,7 +1465,7 @@ The transport has a number of options:
     (no description available)
 
 ``sasl_method``
-
+    (no description available)
 
 ``connection_name``
     For custom connection names (requires at least version 1.10 of the PHP AMQP