include better examples of ParseOptions use in the parsing tutorial

See https://github.com/sparklemotion/nokogiri/issues/2201 for evidence that we can do better, especially with respect to:

- default behavior for XML and HTML being to try to recover
- how to use `norecover` and why you might want to do this
- why to use `huge` and tradeoffs
- how to use `Document#errors` to diagnose what's going on
