doc/ffmpeg: Document xerror

Previous version reviewed-by: Lou Logan <lou@lrcd.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/ffmpeg.texi

@@ -1221,6 +1221,9 @@ Discard all frames excepts keyframes.
 Discard all frames.
 @end table
 
+@item -xerror (@emph{global})
+Stop and exit on error
+
 @end table
 
 As a special exception, you can use a bitmap subtitle stream as input: it