Apache's documentation is fairly vast, and can seem daunting if you're not used to it. There are a number of ways in which it looks for files, but the main thing to look for in /etc/httpd/httpd.conf is the "DocumentRoot" directive. It's set by default to /Library/Webserver/Documents.
Note that changing DocumentRoot won't be quite enough; you should also look for the <Directory "/Library/WebServer/Documents">
stuff</Directory> bit and change the string in that tag to your new location. This part is security settings, like the bit mentioned in your post above.
Specific file locations for specific users are loaded in by a separate module, called "mod_userdir". All it does is produce extra file locations, one for each user. By default it's given the parameter "UserDir Sites", so your personal site ends up being "/Users/<your username>/Sites". You could change that to "UserDir Documents" and it would serve up your documents folder, though I don't recommend that! This module changes the default location for all users at once, though, so be careful if you have others using the same machine - they might find their own personal web sites not working any more.
You may actually find it easier to modify Dreamweaver. I don't have it, so I can't say for sure, but I'd bet that it has a preference to set the 'root' file location for the Web server; just set it to /Library/WebServer/Documents.
There is just one more thing - file permissions. httpd.conf is owned by the root user, so you'll need administrator access on your machine enabled to change apache's configuration. The /Library/WebServer/Documents folder is owned by root, but the admin group has write access. If you (the user you normally log in as) installed your OS, you already have admin access. If you're working on a machine managed by an administrator, then you need to ask him or her to add you to the admin group (the admin group is similar to a Windows Power User - you can install software for anyone on the machine, but you can't touch system files).
And, for any complete newbies who might be surfing by and haven't been scared away yet, don't forget to start and stop the Web server (under Sharing in System Preferences) for changes to the apache configuration to take effect.
For further reference, you also have the apache manual installed by default on your machine, just browse
http://localhost/manual/