misc: Update docstring for Callback

devito/ir/iet/nodes.py

@@ -1110,7 +1110,15 @@ class Callback(Call):
         The return type of the callback.
     param_types : str or list of str
         The return type for each argument of the callback.
+
+    Notes
+    -----
+    The reason Callback is an IET type rather than a SymPy type is
+    due to the fact that, when represented at the SymPy level, the IET
+    engine fails to bind the callback to a specific Call. Consequently,
+    errors occur during the creation of the call graph.
     """
+
     def __init__(self, name, retval, param_types):
 
         super().__init__(name=name)

tests/test_iet.py

@@ -137,7 +137,7 @@ def test_callback_cgen():
     code0 = CGen().visit(foo0_arg)
     assert str(code0) == '(void (*)(int))foo0'
 
-    # test nested calls with a Callback as an argument.
+    # Test nested calls with a Callback as an argument.
     call = Call('foo1', [
         Call('foo2', [foo0_arg])
     ])