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).
This was renamed to `HTMLImports.whenReady`

https://github.com/Polymer/polymer-dev/commit/3a1bb6136702b7b5fea0c24bfa764ae3a5f56e08