Updated payloadKey parameter description

Author: sagidlow

modules/cluster-logging-collector-log-forward-syslog.adoc

@@ -123,7 +123,13 @@ You can configure the following for the `syslog` outputs. For more information,
 ** `14` or `console` for the syslog alert log
 ** `15` or `solaris-cron` for the scheduling daemon
 ** `16`–`23` or `local0` – `local7` for locally used facilities
-* payloadKey: The record field to use as payload for the syslog message.
+* Optional. `payloadKey`: The record field to use as payload for the syslog message.
++
+[NOTE]
+====
+Configuring the `payloadKey` parameter prevents other parameters from being forwarded to the syslog.
+====
++
 * rfc: The RFC to be used for sending log using syslog. The default is RFC5424.
 * severity: The link:https://tools.ietf.org/html/rfc5424#section-6.2.1[syslog severity] to set on outgoing syslog records. The value can be a decimal integer or a case-insensitive keyword:
 ** `0` or `Emergency` for messages indicating the system is unusable