plugin/file: document using an explicit origin (#2893)
plugin/file: document using an explicit origin (#2893)

* plugin/file: document using an explicit origin

See #2880

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

* Code review comments

Signed-off-by: Miek Gieben <00b357f94cf3a1145a32703fb42e2d80418ab806@miek.nl>

yongtang/coredns,chrisohaver/coredns,ii/coredns,coredns/coredns,johnbelamaric/coredns,miekg/coredns,johnbelamaric/coredns,miekg/coredns,ii/coredns