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 
Sorry for bikeshedding, but I'd be inclined to describe when you need it (if the Polymer() call is separate from the polymer-element) instead of when you don't.