iio: Documentation change for inkern interface.

This commit- 314be14bb renamed the _st_ functions to loose the bit
that was meant for staging version but forgot to change
the documentation which still have _st_ sprinkled in some of the
places.

Signed-off-by: anish kumar <anish198519851985@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>