ALSA: docs: Update documentation about SG- and vmalloc-buffers

The recent changes simplified the required setup for SG- and vmalloc-
buffers.  Update the documentation accordingly.

Link: https://lore.kernel.org/r/20191105080138.1260-5-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>