implement comment

Signed-off-by: Quentin Remy <quentin.remy@mpi-hd.mpg.de>

gammapy/maps/core.py

@@ -1031,6 +1031,7 @@ def interp_to_geom(self, geom, preserve_counts=False, fill_value=0, **kwargs):
             the map is a differential quantity (e.g. intensity). Default is False.
         fill_value : float, optional
             The value to use for points outside the interpolation domain.
+            If None, values outside the domain are extrapolated.
             Default is 0.
         **kwargs : dict, optional
             Keyword arguments passed to `Map.interp_by_coord`.