Revised APT-Offline A-Z

I revised the introductory sections (up to "> Set-operation details") of APT-offline A-Z, to clarify and otherwise enhance them. I was frankly disgusted that there was still so much bad writing. Naturally it's better, but I've learned to avoid predicting whether it will need more revisions. At least the introductory section, which is the key to putting the rest into perspective, is fairly good at this point.