[media] DocBook media: fix the poll() 'no QBUF' documentation

Clarify what poll() returns if STREAMON was called but not QBUF.
Make explicit the different behavior for this scenario for
capture and output devices.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/v4l/func-poll.xml

@@ -44,10 +44,18 @@ Capture devices set the <constant>POLLIN</constant> and
 flags. When the function timed out it returns a value of zero, on
 failure it returns <returnvalue>-1</returnvalue> and the
 <varname>errno</varname> variable is set appropriately. When the
-application did not call &VIDIOC-QBUF; or &VIDIOC-STREAMON; yet the
+application did not call &VIDIOC-STREAMON; the
 <function>poll()</function> function succeeds, but sets the
 <constant>POLLERR</constant> flag in the
-<structfield>revents</structfield> field.</para>
+<structfield>revents</structfield> field. When the
+application has called &VIDIOC-STREAMON; for a capture device but hasn't
+yet called &VIDIOC-QBUF;, the <function>poll()</function> function
+succeeds and sets the <constant>POLLERR</constant> flag in the
+<structfield>revents</structfield> field. For output devices this
+same situation will cause <function>poll()</function> to succeed
+as well, but it sets the <constant>POLLOUT</constant> and
+<constant>POLLWRNORM</constant> flags in the <structfield>revents</structfield>
+field.</para>
 
     <para>When use of the <function>read()</function> function has
 been negotiated and the driver does not capture yet, the