media: docs: add GPLv2 for uAPI guide

Newer uAPI docs should be dual-licensed with both GPL
and GFDL. Add a text adding the GPLv2 license text there,
in order to be coherent with the included docs.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/userspace-api/media/index.rst

@@ -37,3 +37,20 @@ Linux Media Infrastructure userspace API
   any later version published by the Free Software Foundation, with no
   Invariant Sections. A copy of the license is included in the chapter
   entitled "GNU Free Documentation License".
+
+Please notice that some documents inside the media userspace API,
+when explicitly mentioned on its source code, are dual-licensed
+with GNU Free Documentation License  Version 1.1 and with the
+GNU General Public License::
+
+  This documentation is free software; you can redistribute it and/or modify it
+  under the terms of the GNU General Public License as published by the Free
+  Software Foundation; either version 2 of the License, or (at your option) any
+  later version.
+
+  This program is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+  FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for
+  more details.
+
+  For more details see the file COPYING in the source distribution of Linux.