Merge pull request #8409 from gyuho/2

dev-guide: note v2 keys to v3 API incompatibility

Documentation/dev-guide/interacting_v3.md

@@ -2,7 +2,8 @@
 
 Users mostly interact with etcd by putting or getting the value of a key. This section describes how to do that by using etcdctl, a command line tool for interacting with etcd server. The concepts described here should apply to the gRPC APIs or client library APIs.
 
-By default, etcdctl talks to the etcd server with the v2 API for backward compatibility. For etcdctl to speak to etcd using the v3 API, the API version must be set to version 3 via the `ETCDCTL_API` environment variable.
+By default, etcdctl talks to the etcd server with the v2 API for backward compatibility. For etcdctl to speak to etcd using the v3 API, the API version must be set to version 3 via the `ETCDCTL_API` environment variable. However note that any key that was created using the v2 API will not be able to be queried via the v3 API.  A v3 API ```etcdctl get``` of a v2 key will exit with 0 and no key data, this is the expected behaviour.
+
 
 ```bash
 export ETCDCTL_API=3