http://github.com/nex3/arc/blob/master/help/arc.arc is from Anarki, a community-maintained branch of "vanilla" Arc. They wrote such a help function, but there are also other extensions and differences that are reflected in said help.
If you're new to Lisp, http://ycombinator.com/arc/tut.txt isn't a bad start. But there are countless Lisp tutorials out there.
As for online documentation: http://files.arcfn.com/doc/ + http://www.arcfn.com/2009/06/whats-new-in-arc3.html; see also http://arclanguage.org/item?id=12228, wherein I technically have another suggestion for learning Arc without the web ;).
[ Haven't dug in very far yet but Arc tutorial appears to be web development oriented, and you could look at the HN source code ]