Document the subset of java.* classes/methods that work in Scala.js

[lihaoyi]

It would be nice to know what works without having to read the source code, and the list has grown relatively large by this point so it's hard to keep track in your head.

We could probably just regex through the stuff in https://github.com/scala-js/scala-js and gather a list of class/defs, which would make it trivial to keep the list up to date: just re-run the regex and regenerate the list each release.