[tldoc] dvipdfmx/gs news for TL21

[Karl Berry]

Sorry about this, but we discovered a problem whose fix has enough
user impact that it seemed necessary to put in the manual. I've just
added this \item to texlive-en.tex, under Dvipdfmx:

  \item Ghostscript is now invoked safely by default; to override
      (assuming all input files are trusted), use
      \verb|-i dvipdfmx-unsafe.cfg|. To use PSTricks with \XeTeX, this is
      required, as in:
      \verb|xetex -output-driver="xdvipdfmx -i dvipdfmx-unsafe.cfg -q -E" ...|

I don't expect to make any more changes to the manual.

Thanks,
Karl