Document inferred name in Polymer() call

This replaces #530, which got a little confused.

Rewriting to reflect the new support for inferred names nearly everywhere required more rewriting than I thought.

PTAL, @sjmiles @sorvell 
@sjmiles So we _modify_ the prototype object, but we also add a polymer base prototype object into the chain... What's the distinction between the stuff we add to the base prototype and the stuff we add/modify on the prototype itself?