Documentation:reference notifiers.txt in freezing-of-tasks.txt

freezing-of-tasks.txt mentions firmware issues without mentioning the use
of the new notifier API to overcome them.  Here's an update.

Signed-off-by: Oliver Neukum <oneukum@suse.de>
Acked-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Nigel Cunningham <nigel@nigel.suspend2.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>