There's a bit of guesswork on my part here because I have not used OS X Server.
Obviously you've found httpd.conf.
The root directory of the standard web server is defined in there - on OS X it is:
DocumentRoot "/Library/WebServer/Documents"
<Directory "/Library/WebServer/Documents">
...
Require all granted <-------------- means anyone can access it from any domain ----------------->
</Directory>
The instructions to which you are referring I guess are the ones about allowing users to set up their own sites, needing the relevant modules to be activated in that file:
#LoadModule userdir_module libexec/apache2/mod_userdir.so
to
LoadModule userdir_module libexec/apache2/mod_userdir.so
#Include /private/etc/apache2/extra/httpd-userdir.conf
to
Include /private/etc/apache2/extra/httpd-userdir.conf
...then in http-userdir.conf....
#Include /private/etc/apache2/users/*.conf
to
Include /private/etc/apache2/users/*.conf
At this point I am assuming that Apache doesn't really care that this is a user-related issue and so any file ending in .conf in that directory will be included in the Apache Server config (once stopped and started). However, the fact the the site name is then ~shortname suggests the user model is important and access permissions may mean you need to be logged in as that user for all I know.
....in the shortname.conf file in that /private/etc/apache2/users/ directory would be something like this:
<Directory "/Users/shortname/Sites/">
AddLanguage en .en
LanguagePriority en fr de
ForceLanguagePriority Fallback
Options Indexes MultiViews
AllowOverride None
Order allow,deny
Allow from localhost
Require all granted
</Directory>
The relevant directory is /Users/shortname/Sites/ ---- I suggest putting a simple index.html page there to test first.
....then http://localhost/~shortname
.....should work.
Remove the line Allow from localhost and access from elsewhere should also be possible.
I wish I could be more help - but without a server to use and check what I am saying I think that is about all I can offer, and I suspect none of this is new to you.
Andy