docs: gpio: explain GPIOD_OUT_* values and toggling active low

I was confused about the gpiod_flags values and thought that
GPIOD_OUT_LOW and GPIOD_OUT_HIGH set the line to be active low / active
high. This is not true, but I got the misconception because the flags
GPIOD_OUT_*_OPEN_DRAIN do change line configuration and there's a
subchapter about *active low* and *open drain* semantics.

Add an explicit mention that the initial value is a logical value (and
not the line configuration or physical line level). Also add a mention
of the function gpiod_toggle_active_low which was previously missing
from this document.
Signed-off-by: Hannu Hartikainen <hannu@hrtk.in>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>

Documentation/driver-api/gpio/consumer.rst

@@ -72,6 +72,10 @@ for the GPIO. Values can be:
 * GPIOD_OUT_HIGH_OPEN_DRAIN same as GPIOD_OUT_HIGH but also enforce the line
   to be electrically used with open drain.
 
+Note that the initial value is *logical* and the physical line level depends on
+whether the line is configured active high or active low (see
+:ref:`active_low_semantics`).
+
 The two last flags are used for use cases where open drain is mandatory, such
 as I2C: if the line is not already configured as open drain in the mappings
 (see board.txt), then open drain will be enforced anyway and a warning will be
@@ -252,6 +256,8 @@ that can't be accessed from hardIRQ handlers, these calls act the same as the
 spinlock-safe calls.
 
 
+.. _active_low_semantics:
+
 The active low and open drain semantics
 ---------------------------------------
 As a consumer should not have to care about the physical line level, all of the
@@ -309,9 +315,11 @@ work on the raw line value::
 	void gpiod_set_raw_value_cansleep(struct gpio_desc *desc, int value)
 	int gpiod_direction_output_raw(struct gpio_desc *desc, int value)
 
-The active low state of a GPIO can also be queried using the following call::
+The active low state of a GPIO can also be queried and toggled using the
+following calls::
 
 	int gpiod_is_active_low(const struct gpio_desc *desc)
+	void gpiod_toggle_active_low(struct gpio_desc *desc)
 
 Note that these functions should only be used with great moderation; a driver
 should not have to care about the physical line level or open drain semantics.