Default config: use ROUTER_SMARTHOST macro; document

Default config: use ROUTER_SMARTHOST macro; document

Work around the `$host` vs CNAME issue for now by re-specifying the
`tls_sni` value on the example `smarthost_smtp` transport, using the
same macro which we use to turn on use of a smarthost.

Uncomment both dnslookup and smarthost routers by default and let the
macro choose between them.

Bring the documentation of the default configuration closer to
up-to-date, on this issue and others which I spotted while in there.