doc: clarify url.format documentation

The original documentation was slightly confusing. It seemed that the
list of items described the properties of the urlObj object, while it
was actually describing the formatting process. This change makes this
clearer.

Reviewed-by: Chris Dickinson <christopher.s.dickinson@gmail.com>

Cherry-picked-from: https://github.com/joyent/node/commit/0603c8345b7c40f9723277d5ae1f44041627dbab