iio: Add ABI documentation for proximity scan_elements

This patch adds ABI documentation entries for in_proximity_en,
in_proximity_type, and in_proximity_index in the scan_elements directory.
At least one user for these is present, the AS3935 Franklin lightning sensor.
Signed-off-by: Haneen Mohammed <hamohammed.sa@gmail.com>
Reviewed-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

Documentation/ABI/testing/sysfs-bus-iio

@@ -1016,6 +1016,7 @@ What:		/sys/.../iio:deviceX/scan_elements/in_incli_y_en
 What:		/sys/.../iio:deviceX/scan_elements/in_pressureY_en
 What:		/sys/.../iio:deviceX/scan_elements/in_pressure_en
 What:		/sys/.../iio:deviceX/scan_elements/in_rot_quaternion_en
+What:		/sys/.../iio:deviceX/scan_elements/in_proximity_en
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -1032,6 +1033,7 @@ What:		/sys/.../iio:deviceX/scan_elements/in_timestamp_type
 What:		/sys/.../iio:deviceX/scan_elements/in_pressureY_type
 What:		/sys/.../iio:deviceX/scan_elements/in_pressure_type
 What:		/sys/.../iio:deviceX/scan_elements/in_rot_quaternion_type
+What:		/sys/.../iio:deviceX/scan_elements/in_proximity_type
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org
 Description:
@@ -1083,6 +1085,7 @@ What:		/sys/.../iio:deviceX/scan_elements/in_timestamp_index
 What:		/sys/.../iio:deviceX/scan_elements/in_pressureY_index
 What:		/sys/.../iio:deviceX/scan_elements/in_pressure_index
 What:		/sys/.../iio:deviceX/scan_elements/in_rot_quaternion_index
+What:		/sys/.../iio:deviceX/scan_elements/in_proximity_index
 KernelVersion:	2.6.37
 Contact:	linux-iio@vger.kernel.org
 Description: