docs(basic-element): make doc annotations doc-viewer compilant

This commit updates the annotations of the existing docs of
`basic-element`. This is the first step to make usage of
Polymer's `core-doc-viewer` component to get a self generated
website with examples, demos and general documentation about
the component itself.

More info about `core-doc-viewer`:
- http://www.polymer-project.org/docs/start/reusableelements.html
- https://groups.google.com/forum/#!searchin/polymer-dev/docs/polymer-dev/NFQ4hIgZSo4/Wa0fwg2k3lEJ