gpio/ABI: document what is already the case

commit 926b663c
"gpiolib: allow GPIOs to be named" added the ability to
name GPIO lines by an array of names stored in the GPIO
chip. This was in 2009 and has been an ABI since. Let's
document it properly.

Cc: Daniel Silverstone <dsilvers@digital-scurf.org>
Cc: Markus Pargmann <mpa@pengutronix.de>
Cc: Johan Hovold <johan@kernel.org>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/ABI/testing/sysfs-gpio

@@ -16,7 +16,8 @@ Description:
     /sys/class/gpio
 	/export ... asks the kernel to export a GPIO to userspace
 	/unexport ... to return a GPIO to the kernel
-	/gpioN ... for each exported GPIO #N
+	/gpioN ... for each exported GPIO #N OR
+	/<LINE-NAME> ... for a properly named GPIO line
 	    /value ... always readable, writes fail for input GPIOs
 	    /direction ... r/w as: in, out (default low); write: high, low
 	    /edge ... r/w as: none, falling, rising, both