Add top-level API docs section for React.createElement.

I thought it was strange that React.createElement is not documented in the [top level API docs](http://facebook.github.io/react/docs/top-level-api.html). It's mentioned elsewhere in the docs so it appears to be a part of the public interface.

This PR just adds a basic mention of this function in that document. I did sign the facebook CLA under this github username, fwiw.