docs: gpio: explain GPIOD_OUT_* values and toggling active low

I was confused about the gpiod_flags values and thought that
GPIOD_OUT_LOW and GPIOD_OUT_HIGH set the line to be active low / active
high. This is not true, but I got the misconception because the flags
GPIOD_OUT_*_OPEN_DRAIN do change line configuration and there's a
subchapter about *active low* and *open drain* semantics.

Add an explicit mention that the initial value is a logical value (and
not the line configuration or physical line level). Also add a mention
of the function gpiod_toggle_active_low which was previously missing
from this document.

Signed-off-by: Hannu Hartikainen <hannu@hrtk.in>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

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