Documentation: gpio: Reconfiguration with unset direction (uAPI v2)

Update description of reconfiguration rules, adding requirement that a
direction flag be set to enable changing configuration for a line.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Link: https://lore.kernel.org/r/20240626052925.174272-5-warthog618@gmail.com


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>