samples: net: dns_resolve: Clarify that DNS queries aren't immediate

The sample in its default configuration waits for DNS server
configuration via IPv4 or IPv6 specific means. Until this
information is receives, DNS querying won't even start. Make
diagnostic messages explicit of this fact, because if user
doesn't have required services available in the network, the
sample will appear to just hang.

Jira: ZEP-2061

Change-Id: I56a5b7354a2d1239a35d8334ed95a15ebd19254f
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>