Inline documentation

Added some documentation for use-package -- looking up the commentary is just a little painful. 

I'm a bit of a novice with git, and I mixed up my commits -- I'd modified things a few days before, and forgotten, in a way that seemed like a good idea, but wasn't. I have reverted this, but don't know whether I should leave this in or out of the pull request. Hope this doesn't make things a mess for you.