ASoC: jack: Clarify GPIO descriptor lookup in struct snd_soc_jack_gpio doc

Clarify struct snd_soc_jack_gpio documentation for the idx and name fields.
Because name is passed as connection ID to gpiod_get_index() when using GPIO
descriptor defined jack pins it is not only used as a label in debugfs but
also as function name lookup in systems that support functions names for
GPIOs.

Clarify also idx since the index is within the function of the GPIO consumer
device and not within the device itself only.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>