New `getHostsByAddr` function for rDNS lookups

[hcooper]

A new getHostsByAddr function for doing reverse DNS lookups.

It replicates the existing forward DNS function getHostByName, but instead calls net.LookupAddr.

The intended downstream use case is to allow Grafana Loki to resolve the IPs in log messages.

Included changes:

• Test for new function added.
• Docs for new function added & existing function expanded.
• Some auto-formatting in functions.go vscode did - no-op change.

[hcooper]

@mattfarina poke :-)