kafka: comment about error return code for kafka_send function

src/modules/kafka/doc/kafka_admin.xml

@@ -157,6 +157,9 @@ modparam("kafka", "topic", "name=third_topic")
 		<para>
 		  Send a message to a specific topic via Kafka server.
 		</para>
+		<para>
+		  This function returns -1 for all sort of errors. (So execution of script continues)
+		</para>
 		<para>
 		  Parameters:
 		  <itemizedlist>