Document inferred name in Polymer()

Fixes #351. Also removed the tag-name argument from many code excerpts on this page.

Also clarified (I hope) section on adding properties and methods. 

Wording on both of these may be somewhat controversial, so @sjmiles, @sorvell, PTAL.
At this point in the doc, we haven't introduced the proto-object yet... It is synonymous with noscript, true...

Hm. I originally had this section up right after the initial definition, so the parallel was really obvious. Let me think on that.