doc/ffmpeg: slightly rework introductory examples

In particular, add a sentence to introduce the example, and add a
simpler starting example with no options.

Also use different format for input.avi and output.mp4, to convey
that the conversion also works on the container format.

Address issue:
http://trac.ffmpeg.org/ticket/8730

doc/ffmpeg.texi

@@ -49,24 +49,32 @@ Do not mix input and output files -- first specify all input files, then all
 output files. Also do not mix options which belong to different files. All
 options apply ONLY to the next input or output file and are reset between files.
 
+Some simple examples follow.
+
 @itemize
 @item
-To set the video bitrate of the output file to 64 kbit/s:
+Convert an input media file to a different format, by re-encoding media streams:
+@example
+ffmpeg -i input.avi output.mp4
+@end example
+
+@item
+Set the video bitrate of the output file to 64 kbit/s:
 @example
-ffmpeg -i input.avi -b:v 64k -bufsize 64k output.avi
+ffmpeg -i input.avi -b:v 64k -bufsize 64k output.mp4
 @end example
 
 @item
-To force the frame rate of the output file to 24 fps:
+Force the frame rate of the output file to 24 fps:
 @example
-ffmpeg -i input.avi -r 24 output.avi
+ffmpeg -i input.avi -r 24 output.mp4
 @end example
 
 @item
-To force the frame rate of the input file (valid for raw formats only)
-to 1 fps and the frame rate of the output file to 24 fps:
+Force the frame rate of the input file (valid for raw formats only) to 1 fps and
+the frame rate of the output file to 24 fps:
 @example
-ffmpeg -r 1 -i input.m2v -r 24 output.avi
+ffmpeg -r 1 -i input.m2v -r 24 output.mp4
 @end example
 @end itemize