i386, x86-64: show that CONFIG_HOTPLUG_CPU is required for suspend on SMP

It's not sufficiently documented that CONFIG_HOTPLUG_CPU is required for
suspend/hibernation on SMP.

Point out the non-obvious.

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>