iio: ABI: docs: Document Senseair Sunrise ABI

Add documentation for the sysfs attributes of the sunrise_co2 driver.

Signed-off-by: Jacopo Mondi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Cameron <[e-mail]>

Documentation/ABI/testing/sysfs-bus-iio-chemical-sunrise-co2

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+What:		/sys/bus/iio/devices/iio:deviceX/in_concentration_co2_calibration_factory
+Date:		August 2021
+KernelVersion:	5.16
+Contact:	Jacopo Mondi <[e-mail]>
+Description:
+		Writing '1' triggers a 'Factory' calibration cycle.
+
+What:		/sys/bus/iio/devices/iio:deviceX/in_concentration_co2_calibration_background
+Date:		August 2021
+KernelVersion:	5.16
+Contact:	Jacopo Mondi <[e-mail]>
+Description:
+		Writing '1' triggers a 'Background' calibration cycle.
+
+What:		/sys/bus/iio/devices/iio:deviceX/error_status_available
+Date:		August 2021
+KernelVersion:	5.16
+Contact:	Jacopo Mondi <[e-mail]>
+Description:
+		Reading returns the list of possible chip error status.
+		Available options are:
+		- 'error_fatal': Analog front-end initialization error
+		- 'error_i2c': Read/write to non-existing register
+		- 'error_algorithm': Corrupted parameters
+		- 'error_calibration': Calibration has failed
+		- 'error_self_diagnostic': Internal interface failure
+		- 'error_out_of_range': Measured concentration out of scale
+		- 'error_memory': Error during memory operations
+		- 'error_no_measurement': Cleared at first measurement
+		- 'error_low_voltage': Sensor regulated voltage too low
+		- 'error_measurement_timeout': Unable to complete measurement
+
+What:		/sys/bus/iio/devices/iio:deviceX/error_status
+Date:		August 2021
+KernelVersion:	5.16
+Contact:	Jacopo Mondi <[e-mail]>
+Description:
+		Reading returns the current chip error status.