extend documentation for gpiod_set_array() functions

Extend the documentation for the gpiod_set_array() functions and elaborate
a bit on possible use cases.

Signed-off-by: Rojhalat Ibrahim <imr@rtschenk.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/gpio/consumer.txt

@@ -259,6 +259,26 @@ GPIOs belonging to the same bank or chip simultaneously if supported by the
 corresponding chip driver. In that case a significantly improved performance
 can be expected. If simultaneous setting is not possible the GPIOs will be set
 sequentially.
+
+The gpiod_set_array() functions take three arguments:
+	* array_size	- the number of array elements
+	* desc_array	- an array of GPIO descriptors
+	* value_array	- an array of values to assign to the GPIOs
+
+The descriptor array can be obtained using the gpiod_get_array() function
+or one of its variants. If the group of descriptors returned by that function
+matches the desired group of GPIOs, those GPIOs can be set by simply using
+the struct gpio_descs returned by gpiod_get_array():
+
+	struct gpio_descs *my_gpio_descs = gpiod_get_array(...);
+	gpiod_set_array(my_gpio_descs->ndescs, my_gpio_descs->desc,
+			my_gpio_values);
+
+It is also possible to set a completely arbitrary array of descriptors. The
+descriptors may be obtained using any combination of gpiod_get() and
+gpiod_get_array(). Afterwards the array of descriptors has to be setup
+manually before it can be used with gpiod_set_array().
+
 Note that for optimal performance GPIOs belonging to the same chip should be
 contiguous within the array of descriptors.