DOC: lua: add "syslog" to Proxy.get_mode() output

Following previous commit: in this patch we add the "syslog" output as
possible return value for Proxy.get_mode() function since log backend
may now be enumerated from lua with 9a74a6c ("MAJOR: log: introduce log
backends")

doc/lua-api/index.rst

@@ -1140,7 +1140,7 @@ Proxy class
 
   :param class_proxy px: A :ref:`proxy_class` which indicates the manipulated
    proxy.
-  :returns: a string "tcp", "http" or "unknown"
+  :returns: a string "tcp", "http", "syslog" or "unknown"
 
 .. js:function:: Proxy.get_srv_act(px)