text: Title: ENH: Improvement of `attach_cls_member_docstring` to handle inherited `__doc__`
  Body: Currently, the function `attach_cls_member_docstring` retrieves the docstring from `member.__doc__`. This approach, however, fails when the member is inherited from a parent class and the `__doc__` is empty. It is notable that the `help` function can access the parent class's `__doc__`.
  
  For classes like `xorbits` that do not inherit from the `_mars` class, the `help` documentation would also be empty if `member.__doc__` is empty. 
  
  This limitation could potentially obscure valuable information, especially for those relying on `help` to understand the functionalities. Thus, I suggest improving `attach_cls_member_docstring` to manage these instances effectively.
  