New Intro and What's Here for REXML::Element

The introductory Rdoc for class REXML::Element currently is this:

Represents a tagged XML element. Elements are characterized by having children, attributes, and names, and can themselves be children.

I've added an intro that's, shall we say, somewhat more robust?

It includes a What's Here section.
This is merged but not yet released, but may be viewed at

• https://ruby.github.io/rexml/REXML/Element.html