platform/x86: thinkpad_acpi: add documentation for battery charge control

Add a section to the Thinkpad ACPI extras driver documentation detailing
the provided features that may be used to modify battery charge related state.
As of yet, only charge_control_{start,end}_threshold attributes are supported
and documented.

Signed-off-by: Barnabás Pőcze <[e-mail]>
Signed-off-by: Andy Shevchenko <[e-mail]>

Documentation/admin-guide/laptops/thinkpad-acpi.rst

···
 review the laptop's user guide:
 http://www.lenovo.com/shop/americas/content/user_guides/x1carbon_2_ug_en.pdf
 
+Battery charge control
+----------------------
+
+sysfs attributes:
+/sys/class/power_supply/BAT*/charge_control_{start,end}_threshold
+
+These two attributes are created for those batteries that are supported by the
+driver. They enable the user to control the battery charge thresholds of the
+given battery. Both values may be read and set. `charge_control_start_threshold`
+accepts an integer between 0 and 99 (inclusive); this value represents a battery
+percentage level, below which charging will begin. `charge_control_end_threshold`
+accepts an integer between 1 and 100 (inclusive); this value represents a battery
+percentage level, above which charging will stop.
+
+The exact semantics of the attributes may be found in
+Documentation/ABI/testing/sysfs-class-power.
+
 Multiple Commands, Module Parameters
 ------------------------------------