Get rid of annoying Apache warning

For years I put up with Apaches annoying warning on startup about not being able to determine a fully qualified domain name:

Could not reliably determine the server's fully qualified domain name, using 127.0.0.1 for ServerName

I finally looked up how to get rid of it, and despite a few pieces of incorrect advice in forums the solution is to add the following:

ServerName localhost

to /etc/apache2/apache2.conf, or replace localhost with whatever makes sense.