Add user argument to exec_create documentation.

docker/client.py

@@ -521,7 +521,7 @@ class Client(requests.Session):
         return self.exec_start(create_res, detach, tty, stream)
 
     def exec_create(self, container, cmd, stdout=True, stderr=True, tty=False,
-                    user=None, privileged=False):
+                    privileged=False, user=None):
         if utils.compare_version('1.15', self._version) < 0:
             raise errors.InvalidVersion('Exec is not supported in API < 1.15')
         if privileged and utils.compare_version('1.19', self._version) < 0:

docs/api.md

@@ -282,6 +282,7 @@ Sets up an exec instance in a running container.
 * stdout (bool): Attach to stdout of the exec command if true. Default: True
 * stderr (bool): Attach to stderr of the exec command if true. Default: True
 * tty (bool): Allocate a pseudo-TTY. Default: False
+* user (str): User to execute command as. Default: root
 
 **Returns** (dict): A dictionary with an exec 'Id' key.