iio: doc: Describe scale attributes for event thresholds

		value is in raw device units or in processed units (as _raw

		and _input do on sysfs direct channel read attributes).

What:		/sys/.../events/in_accel_scale

What:		/sys/.../events/in_accel_peak_scale

What:		/sys/.../events/in_anglvel_scale

What:		/sys/.../events/in_magn_scale

What:		/sys/.../events/in_rot_from_north_magnetic_scale

What:		/sys/.../events/in_rot_from_north_true_scale

What:		/sys/.../events/in_voltage_scale

What:		/sys/.../events/in_voltage_supply_scale

What:		/sys/.../events/in_temp_scale

What:		/sys/.../events/in_illuminance_scale

What:		/sys/.../events/in_proximity_scale

KernelVersion:	3.21

Contact:	linux-iio@vger.kernel.org

Description:

                Specifies the conversion factor from the standard units

                to device specific units used to set the event trigger

                threshold.

What:		/sys/.../events/in_accel_x_thresh_rising_hysteresis

What:		/sys/.../events/in_accel_x_thresh_falling_hysteresis

What:		/sys/.../events/in_accel_x_thresh_either_hysteresis