docs, ssh: unsupport password auth explicitly
The issue with password auth is tracked in and .
@@ -191,7 +191,10 @@ $ docker -H ssh://example.com ps
```
To use SSH connection, you need to set up `ssh` so that it can reach the
-remote host with public key authentication.
+remote host with public key authentication. Password authentication is not
+supported. If your key is protected with passphrase, you need to set up
+`ssh-agent`.
+
Also, you need to have `docker` binary 18.09 or later on the daemon host.
#### Bind Docker to another host/port or a Unix socket
