Describe ARGS in the documentation

[ViralBShah]

From Patrick's youtube video - describe ARGS in the documentation.

[raggi]

I had a look into the other thing he mentioned, which was being able to do the equivalent of:

if $0 == __FILE__
end

It appears that the values are copied through flmain.c, and so access isn't really available. I've got a bit more learning to do before I might be able to get this working.

[raggi]

I'm incorrect, I hadn't read ui/repl.c yet.

[pao]

@raggi, I've gone ahead and split that over to #572 so we can track it separately. Thanks!

[StefanKarpinski]

Done: JuliaLang/www_old.julialang.org@890c8f6.