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>