Improve slice_cube_window deprecation warning

[rnyb]

Using the method slice_cube_window gives the following warning:

.../xtgeo/surface/regular_surface.py:2398: FutureWarning: This method is 
deprecated and will be removed in xtgeo version 5. It is strongly recommended to use `Cube().compute_attributes_in_window()` instead!

It would be helpful to add the name of method that is being deprecated in the warning message, as this will make it easier for users to know where to update the code.