PM: Add comment describing relationships between PM callbacks to pm.h

The UNIVERSAL_DEV_PM_OPS() macro is slightly misleading, because it
may suggest that it's a good idea to point runtime PM callback
pointers to the same routines as system suspend/resume callbacks
.suspend() and .resume(), which is not the case.  For this reason,
add a comment to include/linux/pm.h, next to the definition of
UNIVERSAL_DEV_PM_OPS(), describing how device PM callbacks are
related to each other.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>