I wrote that wiki page, and sorry if it is confusing.

Otherwise, the section in question means - create a certificate by using a valid-looking domain name. For, example, preferably not supl.a, since that doesn't look valid at all. You can use a real, existing domain name, but since the proxy does not even check, it can just be an imaginary domain too. The part about "control" is just saying preferably don't create a certificate for talk.maemo.org, for example, which is not a domain you own (or are likely to own).

Please edit the wiki to make it less confusing