Improve guidelines for GC protocol implementation for heap types

This is a follow-up to https://github.com/python/cpython/pull/125962 which added some guidelines:

• Use type->tp_alloc instead of PyObject_New and PyObject_GC_New.
• Use type->tp_free instead of PyObject_Free and PyObject_GC_Del.

Those two recommendations were introduced to facilitate adding the Py_TPFLAGS_HAVE_GC flag to a heap type (those types must (should?) implement the GC protocol, at least according to the docs: https://docs.python.org/3/c-api/gcsupport.html#supporting-cycle-detection).

Now, the docs should indicate that:

• type->tp_alloc may call PyObject_GC_Track, depending on Py_TPFLAGS_HAVE_GC. Some users could be surprised by this (like me in https://github.com/python/cpython/pull/138266).
• https://docs.python.org/3.14/c-api/gcsupport.html#supporting-cycle-detection should be updated because it mentions using PyObject_GC_New, which itself mentions using tp_alloc directly. Better to just mention tp_alloc.

What I am actually worried about is:

Constructors for container types must conform to two rules:

• The memory for the object must be allocated using PyObject_GC_New or PyObject_GC_NewVar.
• Once all the fields which may contain references to other containers are initialized, it must call PyObject_GC_Track().

Now, tp_alloc automatically calls PyObject_GC_Track so users won't be able to pre-initialize fields, so I suggest that we mention this.

static PyObject *
object_new(PyTypeObject *type)
{
    T *self = NULL;
    PyObject *f1, *f2, *f3;
    
    f1 = do1();
    if (f1 == NULL) { goto error_pre_init; }
    f2 = do2();
    if (f2 == NULL) { goto error_pre_init; }
    f3 = do3();
    if (f3 == NULL) { goto error_pre_init; }
    
    self = (T *)type->tp_alloc(type, 0);
    if (self == NULL) {
        goto error_pre_init;
    }
    self->f1 = f1;
    self->f2 = f2;
    self->f3 = f3;
    f1 = f2 = f3 = NULL;

    if (finalize(self) < 0) {
        goto error;
    }
    return (PyObject *)self;

error_pre_init:
    Py_XDECREF(f1);
    Py_XDECREF(f2);
    Py_XDECREF(f3);
    return NULL;

error_post_init:
    Py_DECREF(self);
    return NULL;
}

cc @ZeroIntensity

docs.python.org/3.14/c-api/gcsupport.html にある C API のガベージコレクションサポートのドキュメントから始め、特に tp_alloc、PyObject_GC_New、PyObject_GC_Track に関するガイダンスを確認してください。tp_alloc がオブジェクトを追跡するタイミングを説明するようにガイダンスを更新し、適切な場合は PyObject_GC_New ではなく tp_alloc を使用してください。文書化された構築規則に一貫性があれば完了です。