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 
OK. I guess I was confused by how the prototype and prototype chain was constructed. 

The main points I wanted to make (and that don't appear to be obvious to people) is that 1) they have the prototype for the native DOM object on their prototype chain, and 2) we add some stuff to their prototype and/or prototype chain. Sounds like I can do that more simply.