* plugin/etcd: update documention

Make clear(er) how this plugin find records.

Fixes: #4048
Signed-off-by: Miek Gieben <miek@miek.nl>

* Update plugin/etcd/README.md
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>

* Update plugin/etcd/README.md
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>
Co-authored-by: Chris O'Haver <cohaver@infoblox.com>