[media] doc-rst: improve display of notes and warnings

There are several notes and warning mesages in the middle of
the media docbook. Use the ReST tags for that, as it makes
them visually better and hightlights them.

While here, modify a few ones to make them clearer.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst b/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst
index 690034a..ae802f1 100644
--- a/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst
+++ b/Documentation/media/uapi/v4l/vidioc-subdev-g-crop.rst
@@ -35,7 +35,7 @@
 Description
 ===========
 
-    **Note**
+.. note::
 
     This is an :ref:`obsolete` interface and may be removed
     in the future. It is superseded by