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>

include/linux/pinctrl/pinconf-generic.h

@@ -82,8 +82,10 @@
  *	operation, if several modes of operation are supported these can be
  *	passed in the argument on a custom form, else just use argument 1
  *	to indicate low power mode, argument 0 turns low power mode off.
- * @PIN_CONFIG_OUTPUT: this will configure the pin in output, use argument
- *	1 to indicate high level, argument 0 to indicate low level.
+ * @PIN_CONFIG_OUTPUT: this will configure the pin as an output. Use argument
+ *	1 to indicate high level, argument 0 to indicate low level. (Please
+ *	see Documentation/pinctrl.txt, section "GPIO mode pitfalls" for a
+ *	discussion around this parameter.)
  * @PIN_CONFIG_END: this is the last enumerator for pin configurations, if
  *	you need to pass in custom configurations to the pin controller, use
  *	PIN_CONFIG_END+1 as the base offset.