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.
...but you don't need this if the tag-name can be inferred. This seems like a more appropriate place to first mention that.