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>