[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>

Documentation/media/uapi/v4l/dev-subdev.rst

@@ -72,14 +72,14 @@ reported on one (or several) V4L2 device nodes.
 Pad-level Formats
 =================
 
-    **Warning**
+.. warning::
 
-    Pad-level formats are only applicable to very complex device that
+    Pad-level formats are only applicable to very complex devices that
     need to expose low-level format configuration to user space. Generic
     V4L2 applications do *not* need to use the API described in this
     section.
 
-    **Note**
+.. note::
 
     For the purpose of this section, the term *format* means the
     combination of media bus data format, frame width and frame height.