nixos/ssh: Add an example of verbatim keys

This confused someone on SO.

(cherry picked from commit 8352cc9a23a5fea11d467a5b8341b24cd162c62a)

nixos/modules/services/networking/ssh/sshd.nix

@@ -41,6 +41,10 @@ let
           Warning: If you are using <literal>NixOps</literal> then don't use this
           option since it will replace the key required for deployment via ssh.
         '';
+        example = [
+          "ssh-rsa AAAAB3NzaC1yc2etc/etc/etcjwrsh8e596z6J0l7 example@host"
+          "ssh-ed25519 AAAAC3NzaCetcetera/etceteraJZMfk3QPfQ foo@bar"
+        ];
       };
 
       keyFiles = mkOption {