update docstring

nifty_gridder.cc

@@ -836,6 +836,8 @@ idx: np.array((nvis,), dtype=np.uint32)
     the indices for the entries to be gridded
 vis: np.array((nvis,), dtype=np.complex)
     The visibility data for the index array
+grid_in: np.array((nu,nv), dtype=np.float64), optional
+    If present, the result is added to this array.
 
 Returns
 =======