Update comments

Author: XuehaiPan

tests/test_with_catch/test_subinterpreter.cpp

@@ -386,10 +386,18 @@ TEST_CASE("gil_safe_call_once_and_store per-interpreter isolation") {
         py::object sub_ordered_dict_type = get_ordered_dict_type_object();
         py::object sub_default_dict_type = get_default_dict_type_object();
 
-        REQUIRE(sub_dict_type.is(dict_type));                       // dict is a static type
-        REQUIRE(sub_ordered_dict_type.is(ordered_dict_type));       // OrderedDict is a static type
+        // Verify that the subinterpreter has its own cached type objects.
+        // For static types, they should be the same object across interpreters.
+        // See also: https://docs.python.org/3/c-api/typeobj.html#static-types
+        REQUIRE(sub_dict_type.is(dict_type));                 // dict is a static type
+        REQUIRE(sub_ordered_dict_type.is(ordered_dict_type)); // OrderedDict is a static type
+        // For heap types, they are dynamically created per-interpreter.
+        // See also: https://docs.python.org/3/c-api/typeobj.html#heap-types
         REQUIRE_FALSE(sub_default_dict_type.is(default_dict_type)); // defaultdict is a heap type
 
+        // Set up a weakref callback to detect when the subinterpreter's cached default_dict_type
+        // is destroyed so the gil_safe_call_once_and_store storage is not leaked when the
+        // subinterpreter is shutdown.
         (void) py::weakref(sub_default_dict_type,
                            py::cpp_function([&](py::handle weakref) -> void {
                                sub_default_dict_type_destroyed = true;