Ok... The documents suck. I think we all agree. Well, now. They don't actually suck, but there's a missing overview/general introduction. I'm going to try, and explain, as a Drupal newb (Linux since Slackware .89, or some such), but LAMP expert.
First, you need to know that Drupal does way more to help you than you think in making multiple sites work off a single installation. That's part of the problem with the docs. It's assumed you know this stuff. The key bit is that Drupal looks for sub-site subdirectories based on the inbound host/domain name, and uses the appropriate one, if found. You don't have to do anything other than set these things up so they work.
Second, you only need to set up the Apache virtual hosts to point at the Drupal installation we're talking about. Read the Apache docs on the VirtualHost, and ServerAlias directives. You may have to read up on DNS configurations to get your domain names to point at the proper IP. 99.9999999% of the time, when you register the domain name, you can take care of these both. In any case, it's probably going to be your service provider in a commercial environment, and your systems administrator in other environments. If you're administering your own box, or test box, like me, you may have some reading to do.
Let's assume you have Drupal working properly on an Apache site with a domain name of example.com. You'd better get this far before trying to get the multi-site thing going.