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>

drivers/platform/x86/Kconfig

@@ -231,6 +231,11 @@ config THINKPAD_ACPI
 
 	  This driver was formerly known as ibm-acpi.
 
+	  Extra functionality will be available if the rfkill (CONFIG_RFKILL)
+	  and/or ALSA (CONFIG_SND) subsystems are available in the kernel.
+	  Note that if you want ThinkPad-ACPI to be built-in instead of
+	  modular, ALSA and rfkill will also have to be built-in.
+
 	  If you have an IBM or Lenovo ThinkPad laptop, say Y or M here.
 
 config THINKPAD_ACPI_ALSA_SUPPORT