[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>

Documentation/sound/alsa/OSS-Emulation.txt

@@ -278,6 +278,21 @@ current mixer configuration by reading and writing the whole file
 image.
 
 
+Duplex Streams
+==============
+
+Note that when attempting to use a single device file for playback and
+capture, the OSS API provides no way to set the format, sample rate or
+number of channels different in each direction.  Thus
+	io_handle = open("device", O_RDWR)
+will only function correctly if the values are the same in each direction.
+
+To use different values in the two directions, use both
+	input_handle = open("device", O_RDONLY)
+	output_handle = open("device", O_WRONLY)
+and set the values for the corresponding handle.
+
+
 Unsupported Features
 ====================