use "example.com" instead of "default" as server name in the various

use "example.com" instead of "default" as server name in the various
examples: it matches the sample config and avoids tricking the user into
thinking that "default" as server name has a special meaning.

While here, document also that http uses the first server matching the
listening port when there's no match on the server name.
(was corrected on this by Crystal Kolipe, thanks)

ok kn on a previous version, improvements from sthen and jmc, ok sthen