avplay: add examples of how to specify size/pixel format through private options

Clarify with an example what the user should use instead of the
deprecated (and no longer working) -s/-pix_fmt options.
Signed-off-by: Anton Khirnov <anton@khirnov.net>

doc/avplay.texi

@@ -39,7 +39,8 @@ Force displayed width.
 Force displayed height.
 @item -s @var{size}
 This option has been removed. Use private format options for specifying the
-input video size.
+input video size.  For example with the rawvideo demuxer you need to specify the
+option @var{video_size}.
 @item -an
 Disable audio.
 @item -vn
@@ -70,7 +71,8 @@ also sources and sinks).
 @table @option
 @item -pix_fmt @var{format}
 This option has been removed. Use private options for specifying the
-input pixel format.
+input pixel format. For example with the rawvideo demuxer you need to specify
+the option @var{pixel_format}.
 @item -stats
 Show the stream duration, the codec parameters, the current position in
 the stream and the audio/video synchronisation drift.