docs: gpio: Fix formatting in description of gpiod_*_array_* functions

The gpiod_*_array_* functions take four arguments, not three.
Additionally, the formatting of the "value_bitmap" line results in
misformatted HTML, so fix that.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>