[latexrefman] Some packages need to run external system commands.

[Vincent Belaïche]

Read in node «\write18».

I find the wording awkward, because what is meant --- IMHO --- is
external wrt LaTeX. But in the context of an OS shell the word
«external» has another meaning, an external command is a command that
is implemented through a program file that you have somewhere on a
harddrive in a directory in the PATH. «external» word is used by
contrast to shell functions and shell builtins.

I propose to fix this by just removing word «external».

Is everybody OK with that ?