doc: document watchdog_device parameter

Commit 8d8d4a936ab73d8449a3574f969b17a90ef9428e introduced the
configuration parameter resources.watchdog_device. This commit
introduces the resources section and watchdog_device parameter in
corosync.conf.5.

Signed-off-by: Adrian Vondendriesch <adrian.vondendriesch@credativ.de>
Reviewed-by: Jan Friesse <jfriesse@redhat.com>