DOC: spoe: Clarify use of the event directive in spoe-message section

Only one event is possible for a spoe-message section. If defined several
time, only the last one is considered. The documentation is now explicit on
this point.

This patch is related to the the issue #1351.
diff --git a/doc/SPOE.txt b/doc/SPOE.txt
index 2330c49..9a4cc4b 100644
--- a/doc/SPOE.txt
+++ b/doc/SPOE.txt
@@ -514,7 +514,8 @@
 event <name> [ { if | unless } <condition> ]
   Set the event that triggers sending of the message. It may optionally be
   followed by an ACL-based condition, in which case it will only be evaluated
-  if the condition is true.
+  if the condition is true. A SPOE message can only be sent on one event. If
+  several events are defined, only the last one is considered.
 
   ACL-based conditions are executed in the context of the stream that handle
   the client and the server connections.