doc: update examples to use -b:v or -b:a

Fixes Ticket613
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -37,7 +37,7 @@ which should be specified first.
 @item
 To set the video bitrate of the output file to 64kbit/s:
 @example
-ffmpeg -i input.avi -b 64k output.avi
+ffmpeg -i input.avi -b:v 64k output.avi
 @end example
 
 @item
@@ -315,7 +315,7 @@ Requires -bufsize to be set.
 Set min video bitrate (in bit/s).
 Most useful in setting up a CBR encode:
 @example
-ffmpeg -i myfile.avi -b 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v
+ffmpeg -i myfile.avi -b:v 4000k -minrate 4000k -maxrate 4000k -bufsize 1835k out.m2v
 @end example
 It is of little use elsewise.
 @item -bufsize @var{size}
@@ -951,7 +951,7 @@ the Linux player does not seem to be very fast, so it can miss
 frames. An example is:
 
 @example
-ffmpeg -g 3 -r 3 -t 10 -b 50k -s qcif -f rv10 /tmp/b.rm
+ffmpeg -g 3 -r 3 -t 10 -b:v 50k -s qcif -f rv10 /tmp/b.rm
 @end example
 
 @item
@@ -1088,7 +1088,7 @@ You can encode to several formats at the same time and define a
 mapping from input stream to output streams:
 
 @example
-ffmpeg -i /tmp/a.wav -map 0:a -b 64k /tmp/a.mp2 -map 0:a -b 128k /tmp/b.mp2
+ffmpeg -i /tmp/a.wav -map 0:a -b:a 64k /tmp/a.mp2 -map 0:a -b:a 128k /tmp/b.mp2
 @end example
 
 Converts a.wav to a.mp2 at 64 kbits and to b.mp2 at 128 kbits. '-map