Update docs on command templating

Author: takluyver

docs/source/server-process.rst

@@ -28,8 +28,11 @@ pairs.
    * A list of strings that is the command used to start the
      process. The following template strings will be replaced:
 
-     * ``{port}`` the port (or path if :ref:`server-process-unix-socket` is True)
-       that the process should listen on.
+     * ``{port}`` the port that the process should listen on. This will be 0 if
+       it should use a Unix socket instead.
+
+     * ``{unix_socket}`` the path at which the process should listen on a Unix
+       socket. This will be an empty string if it should use a TCP port.
 
      * ``{base_url}`` the base URL of the notebook