[ALSA] More description on duplex streams with OSS emulation

Add paragraph to the OSS document to clarify correct use of duplex streams.

Signed-off-by: Alan Horstmann <gineera@aspect135.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>