img2 encoder: use more descriptive vsync names

Signed-off-by: rogerdpack <rogerpack2005@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

doc/muxers.texi

@@ -641,7 +641,7 @@ The following example shows how to use @command{ffmpeg} for creating a
 sequence of files @file{img-001.jpeg}, @file{img-002.jpeg}, ...,
 taking one image every second from the input video:
 @example
-ffmpeg -i in.avi -vsync 1 -r 1 -f image2 'img-%03d.jpeg'
+ffmpeg -i in.avi -vsync cfr -r 1 -f image2 'img-%03d.jpeg'
 @end example
 
 Note that with @command{ffmpeg}, if the format is not specified with the
@@ -649,12 +649,12 @@ Note that with @command{ffmpeg}, if the format is not specified with the
 format, the image2 muxer is automatically selected, so the previous
 command can be written as:
 @example
-ffmpeg -i in.avi -vsync 1 -r 1 'img-%03d.jpeg'
+ffmpeg -i in.avi -vsync cfr -r 1 'img-%03d.jpeg'
 @end example
 
 Note also that the pattern must not necessarily contain "%d" or
 "%0@var{N}d", for example to create a single image file
-@file{img.jpeg} from the input video you can employ the command:
+@file{img.jpeg} from the start of the input video you can employ the command:
 @example
 ffmpeg -i in.avi -f image2 -frames:v 1 img.jpeg
 @end example