[atlas] Exact semantics of DNS' use_probe_resolver

The DNS-specific option use_probe_resolver is mentioned in
<https://atlas.ripe.net/docs/measurement-creation-api/> but with
little indication of the semantics.

My experience is that some probes (with the default retry=0) return
several responses, with the same "prb_id", but different "from". I
assume it means the probe had several resolvers configured and tried
them all.

Am I correct? It may question some conclusions of
<https://labs.ripe.net/Members/stephane_bortzmeyer/how-many-ripe-atlas-probes-can-resolve-ipv6-only-domain-names>.
Is there an option to switch to the more typical behaviour of a DNS
client (try the first resolver configured and switch to the second if
the first does not reply)?