[ALSA] ASoC documentation updates

Update the ASoC documentation.  Along with minor formatting and grammar
cleanups it moves the ASoC overview into the present tense to reflect
the fact that it has now been merged.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>