gpioset: reword note on post-exit behaviour

gpioset: reword note on post-exit behaviour

The note regarding the state of a line after gpioset exits is confusing
and unhelpful to the average reader, if not outright incorrect.

A common mis-interpretation is that this behaviour is arbitrarily chosen
by spiteful implementors. The note also specifies that the line reverts
to default, but that is not always the case, or is at least out of the
control of gpioset or libgpiod.

Reword the note to constrain the scope to that relevant to the likely
reader, and to emphasize that the behaviour is inherent in the kernel
GPIO interface, not in the gpioset implementation.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>