ACPI: WMI: Improve Kconfig description

As Pavel Machek has pointed out, the Kconfig entry for WMI is pretty
non-descriptive.

Rewrite it so that it explains what ACPI-WMI is, and why anyone
would want to enable it.

Many thanks to Ray Lee for ideas on this.

Signed-off-by: Carlos Corbacho <carlos@strangeworlds.co.uk>
CC: Pavel Machek <pavel@ucw.cz>
CC: Ray Lee <ray-lk@madrabbit.org>
Signed-off-by: Len Brown <len.brown@intel.com>