man: add a description of handling of single-label names

man: add a description of handling of single-label names

It turns out that our man page didn't describe the handling of single-label
names almost at all. This probably adds to the confusion regarding the subject.
So let's first describe what our current implementation is doing.

Quoting https://www.iab.org/documents/correspondence-reports-documents/2013-2/iab-statement-dotless-domains-considered-harmful/:
> Applications and platforms that apply a suffix search list to a single-label
> name are in conformance with IETF standards track RFCs. Furthermore,
> applications and platforms that do not query DNS for a TLD are in conformance
> with IETF standards track recommendations

Current behaviour is in line with that recommendation.

For #13763.