Minor edits to simbad.rst

I read through the SIMBAD docs page:
http://astroquery.readthedocs.org/en/latest/simbad.html

Some of the outputs changed ... e.g. `Simbad.list_votable_fields()` now prints much more info than shown in the tutorial.

In the API docs section, `SimbadResult` is mentioned, but no API docs given ... it should be added?
`ConfigurationItem` should be removed from the API docs section?