Document the new html template tag

[justinfagnani]

See Polymer/polymer#4937 and Polymer/polymer#4971 (review)

[justinfagnani]

I'm anticipating possible confusion when documenting the ability to have expressions in Polymer templates - people may wonder if they can do data-binding there, or how it relates to expressions in something like lit-html. One suggestion for explaining this is to emphasize the static-ness of the property/getter and clarify that the expressions are evaluated just once when the class is defined.