Add comment about scaling constants

Co-authored-by: Andreas Klöckner <inform@tiker.net>

Author: isuruf

pytential/symbolic/pde/system_utils.py

@@ -423,6 +423,7 @@ def get_deriv_relation_kernel(kernel: ExpressionKernel,
         if coeff == 0:
             continue
         if mis[i] != (-1, -1, -1):
+            # account for scaling constants in kernels
             coeff *= _get_sympy_kernel_expression(kernel.global_scaling_const,
                     kernel_arguments)
             coeff /= _get_sympy_kernel_expression(base_kernel.global_scaling_const,