[latexrefman] latexmk as an alternative to write18

[Vincent Belaïche]

Still on node \write18, where sagetex is given as an example.

I think that we should mention that latexmk has example rc scripts for
almost all of this packages like asymptote or sagetex which needs to
execute friend commands. And therefore if you compile with latexmk you
do not need to enable write18 as latexmk will make the proper command
sequence for you.

Personally, I find that using an appropriate latexmk configuration is
both safer and simpler, and I would like to suggest that in the
unofficial refman.

Any comment/brickbats ?
  V.