ENS settings in config are not documented

[haltman-at]

The config reference doesn't include documentation of the ens settings.

It seems that currently the legal ens settings are enabled and registryAddress, although the way that the latter behaves is a little inconsistent at the moment.

Also, in addition to the ens settings, there's also the registry network setting, which isn't documented either (see linked issue above -- the way it works at present is a little inconsistent).

Possibly documentation of this should wait until the linked issue above is fixed so that it all works consistently...?

[haltman-at]

Oops, it turns out there is documentation... but it's not in the reference docs, and it doesn't line up with how the code actually works!