Polymer doc improvements, plus a fix for bug #1071.

Staged at:

https://poly-dot-dart-lang.appspot.com/polymer/

Especially look at the FAQ changes and the details added about dart_support.js:

https://poly-dot-dart-lang.appspot.com/polymer/faq.html#q-what-documentation-exists-for-polymerdart
https://poly-dot-dart-lang.appspot.com/polymer/using-elements/#using-custom-elements-in-html-code

R=sigmund@google.com

Review URL: https://chromiumcodereview.appspot.com/619173002