plugin/file: document wrong behavior in lookup fox Apex (#4376)
plugin/file: document wrong behavior in lookup fox Apex (#4376)

An apex only zone returns the wrong answer, document this by adding a
test.

Closes: #4374

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>
chrisohaver/coredns,miekg/coredns,coredns/coredns,miekg/coredns,yongtang/coredns