[ALSA] writing-an-alsa-driver.tmpl: English style improvements

This patch brings some English style improvements throughout the
document, as well as 1 or 2 extra technical details.

Signed-off-by: Michael Opdenacker <michael@free-electrons.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>