Use shim-shadowdom to get Polymer to fix '^' combinators for non-shadowdom browsers.

https://github.com/Polymer/docs/issues/269
Allow <cxx-example>s inside <cxx-function>s, and stop enumerating the acceptable elements there.
Make examples more flexible as either inline or block-level elements.

This matches the current definition in the Concepts TS.
Prevent <cxx-note> from wrapping inside the begin and end markers.