Add axis to docstring for _replace_nan_test_axis

Author: ndgrigorian

dpnp/dpnp_iface_nanfunctions.py

@@ -105,6 +105,8 @@ def _replace_nan_test_axis(a, val, axis):
         Input array.
     val : float
         NaN values are set to `val` before doing the operation.
+    axis : int
+        Axis along which to test for all-NaN slices.
 
     Returns
     -------