minor gpio doc update

issues including wire-OR and output latencies.

The get/set calls have no error returns because "invalid GPIO" should have

been reported earlier in gpio_set_direction().  However, note that not all

been reported earlier from gpio_direction_*().  However, note that not all

platforms can read the value of output pins; those that can't should always

return zero.  Also, using these calls for GPIOs that can't safely be accessed

without sleeping (see below) is an error.

Those return either the corresponding number in the other namespace, or

else a negative errno code if the mapping can't be done.  (For example,

some GPIOs can't used as IRQs.)  It is an unchecked error to use a GPIO

number that hasn't been marked as an input using gpio_set_direction(), or

number that wasn't set up as an input using gpio_direction_input(), or

to use an IRQ number that didn't originally come from gpio_to_irq().

These two mapping calls are expected to cost on the order of a single