doc: fill documentation for vfwcap

Originally committed as revision 25553 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/indevs.texi

@@ -167,6 +167,10 @@ ffmpeg -f video4linux2 -i /dev/video0 out.mpeg
 
 VfW (Video for Windows) capture input device.
 
+The filename passed as input is the capture driver number, ranging from
+0 to 9. You may use "list" as filename to print a list of drivers. Any
+other filename will be interpreted as device number 0.
+
 @section x11grab
 
 X11 video input device.