Add docs for Polymer helper functions

Address #666, #353, and partially addresses #244 (swiped some material from Addy's doc, but we should get the full doc in here at some point).
We are *not*. D'oh. I made the foolish assumption that since HTMLImports is the namespace of the polyfill, it wasn't loaded in the native case. But apparently it gets stuck in there somehow:

https://github.com/Polymer/HTMLImports/issues/75