Documentation: gpio: Clarify effect of active low flag on line values

Documentation: gpio: Clarify effect of active low flag on line values

The documentation does not make sufficiently clear that the uAPI deals with
logical line values, nor does it describe the influence of the active low
flag on the mapping between physical and logical line values.

Clarify the relationship between physical and logical line values and the
effect of the active low flag for ioctls passing line values.

Suggested-by: David C. Rankin <drankinatty@gmail.com>
Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240610092157.9147-2-warthog618@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>