Example of plotting coordinates on an all-sky map (final pull request)

This should be the final pull request as discussed in issue #2726. There were some problems with my repo, so I had to re-create my branch and so github doesn't allow me to update the original pull request.
It's not actually `SkyCoord` that's doing this, but rather `Angle`.  `SkyCoord` actually has nothing to do with the actually wrapping.  So I'd rephrase this as something like ``This better harnesses the power of the `astropy.coordinates.Angle` object.``  That will then also create a link to the API docs, so that the interested user will be able to go and see what other things `Angle` can do.