[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: NMichael Opdenacker <michael@free-electrons.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>
Signed-off-by: NJaroslav Kysela <perex@perex.cz>