Add a final XML pattern match example, warning about whitespace nodes

These examples have only scratched the surface of what you can do with XML.
There are many other extensions, libraries, and tools you could learn
about, some customized for Scala, some made for Java but usable in
Scala, and some language-neutral. What you should walk away from this
chapter with is how to use semi-structured data for interchange, and how
to access semi-structured data via Scala's XML support.