pinctrl: provide documentation pointer

The PIN_CONFIG_OUTPUT parameter is really tricky to understand
and needs an explicit pointer to the documentation.

Cc: Tomasz Figa <t.figa@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>