Commit e2ae6159 "virtio-serial: report frontend connection state via
monitor" neglected to document the new event is rate-limited.  Fix
that.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200806081147.3123652-3-armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>