[manual] Update of section on apt-setup

[Frans Pop <elendil@planet.nl>]

I've done a fairly large update (r49837) of the section on apt-setup to 
better document the current functionality and especially to help users with 
the question whether they should select a network mirror or not.

A review of and comments on the changes would be appreciated.

Cheers,
FJP

Attachment: signature.asc
Description: This is a digitally signed message part.