Documentation Improvements

Preview:
http://www.stsci.edu/~lbradley/photutils/
Here you use `mask`, but then in the next plot directive you use `~mask`.

Besides fixing this one issue, it would be really nice if there was a way to avoid the code duplication with the need to keep them manually consistent.
I think it's possible to have continued code blocks in Sphinx but haven't used those myself ... anyone has tried them or has a good solution for this?