ACPI: docs: Update the pm_profile sysfs attribute documentation

The list of possible pm_profile values listed in sysfs-acpi-pmprofile
does not include the tablet one (8), so update it.

While at it, rewrite the description so it is more accurate and somewhat
easier to follow.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>