Documentation: gpio: consumer: describe active low property

I've been searching for any documentation of 'the active-low property of a GPIO'
already mentioned in this documenation. But couldn't find any. Add it.

Sigend-off-by: Dirk Behme <dirk.behme@gmail.com>
Acked-by: Alexandre Courbot <acourbot@nvidia.com>
[Spelling, grammar fixes]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/gpio/consumer.txt

@@ -237,6 +237,39 @@ Note that these functions should only be used with great moderation ; a driver
 should not have to care about the physical line level.
 
 
+The active-low property
+-----------------------
+
+As a driver should not have to care about the physical line level, all of the
+gpiod_set_value_xxx() or gpiod_set_array_value_xxx() functions operate with
+the *logical* value. With this they take the active-low property into account.
+This means that they check whether the GPIO is configured to be active-low,
+and if so, they manipulate the passed value before the physical line level is
+driven.
+
+With this, all the gpiod_set_(array)_value_xxx() functions interpret the
+parameter "value" as "active" ("1") or "inactive" ("0"). The physical line
+level will be driven accordingly.
+
+As an example, if the active-low property for a dedicated GPIO is set, and the
+gpiod_set_(array)_value_xxx() passes "active" ("1"), the physical line level
+will be driven low.
+
+To summarize:
+
+Function (example)               active-low proporty  physical line
+gpiod_set_raw_value(desc, 0);        don't care           low
+gpiod_set_raw_value(desc, 1);        don't care           high
+gpiod_set_value(desc, 0);       default (active-high)     low
+gpiod_set_value(desc, 1);       default (active-high)     high
+gpiod_set_value(desc, 0);             active-low          high
+gpiod_set_value(desc, 1);             active-low          low
+
+Please note again that the set_raw/get_raw functions should be avoided as much
+as possible, especially by drivers which should not care about the actual
+physical line level and worry about the logical value instead.
+
+
 Set multiple GPIO outputs with a single function call
 -----------------------------------------------------
 The following functions set the output values of an array of GPIOs: