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 
Also pendantic, but in my mental model, these are not alternative _ways_ to register. The only way to register without  calling Polymer() is with `noscript`. It's probably not worth an action item now, but maybe next time you pass by consider discussing only in terms of when/where the code that calls `Polymer()` can live.