Documentation: gpio: fix typo and unclear legacy API section

The "Interacting With the Legacy GPIO Subsystem" of the documentation
was unclear at best, and even included a sentence that seems to say the
opposite of what it should say about the lifetime of the return value of
the conversion functions.

Try to clarify things a bit and hopefully make that section more
readable.

Reported-by: Andy Shevchenko <andy.shevchenko@gmail.com>
BugLink: https://stackoverflow.com/q/64455505/2511795
Signed-off-by: Alexandre Courbot <gnurou@gmail.com>
Link: https://lore.kernel.org/r/20201122092548.61979-1-gnurou@gmail.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/driver-api/gpio/consumer.rst

@@ -440,18 +440,20 @@ For details refer to Documentation/firmware-guide/acpi/gpio-properties.rst
 
 Interacting With the Legacy GPIO Subsystem
 ==========================================
-Many kernel subsystems still handle GPIOs using the legacy integer-based
+Many kernel subsystems and drivers still handle GPIOs using the legacy
-interface. Although it is strongly encouraged to upgrade them to the safer
+integer-based interface. It is strongly recommended to update these to the new
-descriptor-based API, the following two functions allow you to convert a GPIO
+gpiod interface. For cases where both interfaces need to be used, the following
-descriptor into the GPIO integer namespace and vice-versa::
+two functions allow to convert a GPIO descriptor into the GPIO integer namespace
+and vice-versa::
 
 	int desc_to_gpio(const struct gpio_desc *desc)
 	struct gpio_desc *gpio_to_desc(unsigned gpio)
 
-The GPIO number returned by desc_to_gpio() can be safely used as long as the
+The GPIO number returned by desc_to_gpio() can safely be used as a parameter of
-GPIO descriptor has not been freed. All the same, a GPIO number passed to
+the gpio\_*() functions for as long as the GPIO descriptor `desc` is not freed.
-gpio_to_desc() must have been properly acquired, and usage of the returned GPIO
+All the same, a GPIO number passed to gpio_to_desc() must first be properly
-descriptor is only possible after the GPIO number has been released.
+acquired using e.g. gpio_request_one(), and the returned GPIO descriptor is only
+considered valid until that GPIO number is released using gpio_free().
 
 Freeing a GPIO obtained by one API with the other API is forbidden and an
 unchecked error.