Update comments

Author: XuehaiPan

tests/test_custom_type_setup.cpp

@@ -64,7 +64,10 @@ void add_gc_checkers_with_weakrefs(const py::object &obj) {
 
 TEST_SUBMODULE(custom_type_setup, m) {
     py::class_<ContainerOwnsPythonObjects> cls(
-        m, "ContainerOwnsPythonObjects", py::custom_type_setup([](PyHeapTypeObject *heap_type) {
+        m,
+        "ContainerOwnsPythonObjects",
+        // Please review/update docs/advanced/classes.rst after making changes here.
+        py::custom_type_setup([](PyHeapTypeObject *heap_type) {
             auto *type = &heap_type->ht_type;
             type->tp_flags |= Py_TPFLAGS_HAVE_GC;
             type->tp_traverse = [](PyObject *self_base, visitproc visit, void *arg) {

tests/test_custom_type_setup.py

@@ -58,7 +58,10 @@ def test_indirect_cycle(gc_tester):
 )
 @pytest.mark.skipif("env.PYPY or env.GRAALPY")
 def test_py_cast_useable_on_shutdown():
-    """Test that py::cast works during interpreter shutdown."""
+    """Test that py::cast works during interpreter shutdown.
+
+    See PR #5972 and https://github.com/pybind/pybind11/pull/5958#discussion_r2717645230.
+    """
     env.check_script_success_in_subprocess(
         f"""
         import sys