pm: document use of RTC in pm_trace

As pm_trace uses the system's hardware clock to save its magic value,
users of that option should be warned that using this debug option will
result in an incorrect system time after resume.

Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>