DOC: Add more detailed description of commit prefixes

I was unfamiliar with these particular prefixes when I just contributed, so I thought it might be helpful for contributors to have some general guidelines for when to use them.  I honestly just guessed at some of them, so if they are wrong please feel free to correct them.

However, I looked on the mailing list and didn't see any guidelines there either, so I think it would be nice to enumerate them here.