[media] DocBook: mark SDR API as Experimental

Let it be experimental still as all SDR drivers are in staging.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: NAntti Palosaari <crope@iki.fi>
Acked-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <m.chehab@samsung.com>

Documentation/DocBook/media/v4l/compat.xml

@@ -2661,6 +2661,9 @@ ioctls.</para>
         <listitem>
 	  <para>Exporting DMABUF files using &VIDIOC-EXPBUF; ioctl.</para>
         </listitem>
+        <listitem>
+	  <para>Software Defined Radio (SDR) Interface, <xref linkend="sdr" />.</para>
+        </listitem>
       </itemizedlist>
     </section>
 

Documentation/DocBook/media/v4l/dev-sdr.xml

   <title>Software Defined Radio Interface (SDR)</title>
 
+  <note>
+    <title>Experimental</title>
+    <para>This is an <link linkend="experimental"> experimental </link>
+    interface and may change in the future.</para>
+  </note>
+
   <para>
 SDR is an abbreviation of Software Defined Radio, the radio device
 which uses application software for modulation or demodulation. This interface