ALSA: doc: Brush up the old writing-an-alsa-driver

Slightly brushing up and throw the old dust away from my ancient
writing-an-alsa-driver document.  The contents aren't changed so much
but the obsoleted parts are dropped.

Also, remove the date and the version number.  It's useless.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>