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 <pobrn@protonmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>