Make a distinction between sphinx search and sphinx python

I was left scratching my head just now as to why `brew install sphinx` wasn't installing [sphinx](http://sphinx-doc.org/)

I've added a note to the `caveats` of the sphinx formula, but perhaps there's a better way of doing this? I'd argue that the formula should be renamed to `sphinx-search`

(OK, maybe I should have looked at the URL first, but it would be nice if it was a little clearer)