corosync.conf.5: watchdog support is conditional

Signed-off-by: Ferenc Wágner <wferi@debian.org>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>

man/corosync.conf.5

@@ -652,6 +652,8 @@ directive it is possible to specify options for resources.
 Possible option is:
 .TP
 watchdog_device
+(Valid only if Corosync was compiled with watchdog support.)
+.br
 Watchdog device to use, for example /dev/watchdog.
 If unset, empty or "off", no watchdog is used.
 .IP