Adding docstring to grid.negate_depth()

Author: erikvansebille

parcels/grid.py

@@ -115,6 +115,10 @@ def depth(self):
         return self._depth
 
     def negate_depth(self) -> None:
+        """Method to flip the sign of the depth dimension of a Grid.
+        Note that this method does _not_ change the direction of the vertical velocity;
+        for that users need to add a fieldset.W.set_scaling_factor(-1.0)
+        """
         self._depth = -self._depth
 
     @property