driver: gpio: remove documentation related to pin-based callback config

This reverts the documentation component of commit
eb6ea286.

The original change broke the API contract: drivers that use GPIOs need
to be able to configure callbacks without being aware of whether a
particular implementation expects to use a mask or a pin ordinal.

Revert the API documentation to its original format, and mark that the
added field should be removed when issue #11565 is resolved.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>