iio: ina2xx: add ABI documentation entry sysfs-bus-iio-ina2xx-adc

Documentation for attributes:

 * in_allow_async_readout
 * in_shunt_resistor
Signed-off-by: Marc Titinger <mtitinger@baylibre.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio-ina2xx-adc

+What:		/sys/bus/iio/devices/iio:deviceX/in_allow_async_readout
+Date:		December 2015
+KernelVersion:	4.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		By default (value '0'), the capture thread checks for the Conversion
+		Ready Flag to being set prior to committing a new value to the sample
+		buffer. This synchronizes the in-chip conversion rate with the
+		in-driver readout rate at the cost of an additional register read.
+
+		Writing '1' will remove the polling for the Conversion Ready Flags to
+		save the additional i2c transaction, which will improve the bandwidth
+		available for reading data. However, samples can be occasionally skipped
+		or repeated, depending on the beat between the capture and conversion
+		rates.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_shunt_resistor
+Date:		December 2015
+KernelVersion:	4.4
+Contact:	linux-iio@vger.kernel.org
+Description:
+		The value of the shunt resistor may be known only at runtime fom an
+		eeprom content read by a client application. This attribute allows to
+		set its value in ohms.