Added note on PML with fractal distributions

docs/source/input_api.rst

@@ -175,6 +175,10 @@ Fractal Box
 -----------
 .. autoclass:: gprMax.cmds_geometry.fractal_box.FractalBox
 
+.. note::
+
+    * Currently (2024) we are not aware of a formulation of Perfectly Matched Layer (PML) absorbing boundary that can specifically handle distributions of material properties (such as those created by fractals) throughout the thickness of the PML, i.e. this is a required area of research. Our PML formulations can work to an extent depending on your modelling scenario and requirements. You may need to increase the thickness of the PML and/or consider tuning the parameters of the PML (:ref:`pml-tuning`) to improve performance for your specific model.
+
 Add Grass
 ---------
 .. autoclass:: gprMax.cmds_geometry.add_grass.AddGrass