Update XGrid localize docstring

Author: VeckoTheGecko

parcels/xgrid.py

@@ -137,6 +137,8 @@ def localize(self, position: dict[_XGRID_AXES, tuple[int, float]], dims: list[st
 
         Uses dimensions of the DataArray to determine the staggered grid.
 
+        WARNING: This API is unstable and subject to change in future versions.
+
         Parameters
         ----------
         position : dict