iio: Add ABI documentation for proximity
iio: Add ABI documentation for proximity

has been in use for a long time, but never documented
proximity is unit-less, except otherwise stated, see e.g.
the as3935 driver where it is meter

Signed-off-by: Peter Meerwald <a2c19d57d338d8f9d8de092166164455662a8cbe@pmeerw.net>
Signed-off-by: Jonathan Cameron <09f65b71b7655725897b2fd41a09a0cefe2e1ace@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs