Fix docstring on rolling_window

[santisoler]

Description of the desired feature

The spacing parameter of the rolling_window function controls the distance between the center of two adjacent windows.
Nevertheless, the docstring reads:

    spacing : float, tuple = (s_north, s_east), or None
        The window size in the South-North and West-East directions,
        respectively. A single value means that the size is equal in both
        directions.

Which is very confusing, given that the size parameter is the ones that controls the size of the windows.

We should fix the documentation so it reflects the actual role of the spacing parameter.

Are you willing to help implement and maintain this feature? Yes