Add documentation for the @js functions and friends

[NHDaly]

Part of #160.

[NHDaly]

I would actually really appreciate someone's review on this one before I merge it! :)

Thanks!

[pfitzseb]

I feel like "...converted to javascript..." warrants some more explanation, but I'm not sure if those docstrings are the best place for that. LGTM otherwise.

[NHDaly]

Hmm agreed. Would it be too much to add something like this small paragraph to both of those?:

`expr` will be parsed as julia code, and then converted directly to the
equivalent javascript. Language keywords that don't exist in julia can be
represented with their macro equivalents, `@var`, `@new`, etc.

[NHDaly]

Okay cool! :) I've just added that, and i'll merge it now. We can always change the docstring later! :)