The corresponding documentation hasn't been pushed yet; I'll post a link when it is up.
Note that XLA is work in progress --- we're releasing the code early because we want to get the community involved. The GPU backend is in good shape, and improving by the day. We haven't had as much time to devote to the CPU backend, and it only has limited support for parallelism. Contributions welcome!
Unofficial answer: no promises, but it should be open-source soon. It may even be released in the next day or two. Watch this space!