thinkpad-acpi: improve Kconfig help text

Document that rfkill and ALSA functionality exists, but requires the
subsystems to be available, and not modular if thinkpad-acpi is not
modular.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>