dns: do not indicate invalid IPs are IPv6

In lib/dns.js, use `isIP()` instead of `isIPv4()` for determining the
`family` property in `lookup()`. If an invalid IP address is returned,
the `family` currently provided is `6`. With this change, it will be
`0`. Update documentation to reflect this.

PR-URL: https://github.com/nodejs/node/pull/27081


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>