Update pydoc for container.execute (#356)

Update the pydoc for container's execute method to clarify the return
value.

Signed-off-by: Adam Israel <adam@adamisrael.com>

pylxd/models/container.py

@@ -382,7 +382,7 @@ def execute(
             message recived via stderr
         :type stderr_handler: Callable[[str], None]
         :raises ValueError: if the ws4py library is not installed.
-        :returns: The return value, stdout and stdin
+        :returns: A tuple of `(exit_code, stdout, stderr)`
         :rtype: _ContainerExecuteResult() namedtuple
         """
         if not _ws4py_installed: