Part of the Arc philosophy is to implement the language in such a clear, concise way that it serves as its own specification--or at least, that any proper specification would be harder/longer to read than the implementation itself. If an implementation is fine for what you're doing, there's a very recent thread (http://arclanguage.org/item?id=11583) about getting started programming in Arc. (It's easier to discover recent discussion if you click "new" at the top of the page.)
If it's really a specification you need--maybe you want to write an implementation under a license not compatible with the Artistic License 2.0... or maybe you just like specifications :) --then I think your best bet is the documentation at http://files.arcfn.com/doc/. It isn't perfectly comprehensive and up-to-date, but it is very good, and importantly, it doesn't necessarily expect you to want to look at the source code of the Arc implementation.
