docs: use new java hello. not pingpong. not old java hello.

Mmmostly a bunch of tweaks to: include hello instead of pingpong; describe hello instead of pingpong. Add comments to hello so that Sphinx's include-via-regexp could figure out which parts to include.

While I was in there:

Existing doc said how to run tests, but didn't show a test BUILD target. D'oh. So show one.

New hello app has bundles=bundle(some helper files). So say a little something about bundles=

Bundle results for a java_library changed. used to be a libs/foo.jar , now top-jar has a Foo.class in it. So fix up that text

Fix up rst formatting in tshoot.rst.

Punted on: it's nice that doc talks about bundles, but sample code
doesn't use it in a good way. https://github.com/pantsbuild/pants/issues/110

Testing Done:
Re-reading a week later is like using a fresh pair of eyes, I tell you.

Reviewed at https://rbcommons.com/s/twitter/r/273/