[MPlayer-DOCS] xvidencopts description in manpage is obsolete

[Corey Hickey]

As far as I can tell, the manpage was never updated to reflect support
for xvid 1.0. With ve_xvid4.c, several of the old options are no longer
available, and several new ones are. For example, there used to be an
option "psnr", but now instead there is "stats".

The list of parameters is easy to see starting at line 118 of
ve_xvid4.c. I'm not familiar enough with xvid to know what they all
mean, though, or else I'd send a patch myself.

-Corey