Improves the GPIO API description a little bit.

gpio_pin_max must return the maximum supported pin number and not the total
number of pins on the system.

PR:		157070
Submitted by:	brix


git-svn-id: svn+ssh://svn.freebsd.org/base/head@276168 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f