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 
I would tend to call this a _prototype chain_. A single prototype is an object, a series of connected prototypes is a chain. We do modify the prototype object, but we _assemble_ the chain. 
Also nit picky: I suspect the following bullets are a bit too technical, I'd prefer it to emphasize results and not methodology (i.e. "Element instances have your prototype, a base prototype, and a native prototype in their chain. Features provided by the prototype extensions are documented here [ahem]").