[media] DocBook: add SDR specific info to G_MODULATOR / S_MODULATOR

Add SDR specific notes to G_MODULATOR / S_MODULATOR documentation.
Signed-off-by: Antti Palosaari <crope@iki.fi>
[hans.verkuil@cisco.com: add quotes in "The term 'modulator'"]
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>

Documentation/DocBook/media/v4l/vidioc-g-modulator.xml

@@ -78,6 +78,12 @@ different audio modulation if the request cannot be satisfied. However
 this is a write-only ioctl, it does not return the actual audio
 modulation selected.</para>
 
+    <para><link linkend="sdr">SDR</link> specific modulator types are
+<constant>V4L2_TUNER_SDR</constant> and <constant>V4L2_TUNER_RF</constant>.
+For SDR devices <structfield>txsubchans</structfield> field must be
+initialized to zero.
+The term 'modulator' means SDR transmitter in this context.</para>
+
     <para>To change the radio frequency the &VIDIOC-S-FREQUENCY; ioctl
 is available.</para>