change docstring for zne/layer_scaling

mitiq/zne/scaling/layer_scaling.py

@@ -24,6 +24,15 @@ def layer_folding(
 ) -> cirq.Circuit:
     """Applies a variable amount of folding to select layers of a circuit.
 
+    Note that this method only works for the univariate extrapolation methods.
+    It allows a user to choose which layers in the input circuit will be scaled
+    The layerwise folding required for multivariate extrapolation is different
+    as the layers in the input circuit have to be scaled in a specific pattern.
+    The required specific pattern for multivariate extrapolation does not allow
+    a user to provide a choice of which layers to fold. If you would prefer to
+    use a multivariate extrapolation method for unitary
+    folding, use the functions available in `mitiq/lre` instead.
+
     Args:
         circuit: The input circuit.
         layers_to_fold: A list with the index referring to the layer number,