gh-156995: Add caution around API behavior change

The subtle change between free-threading and GIL-enabled builds resulted
in the `Py_REFCNT(op) == 1` `bytes` object inside `bytearray` was
considered non-unique during `_PyBytes_Resize` which resulted in it
being substitued for an immortal global rather than keeping the
allocation only in free-threaded builds.

The behavior has been fixed in `_PyBytes_Resize` but the hazard exists
and was a part of a release blocking bug so urge caution around
that case when porting.

Author: cmaloney

Doc/c-api/object.rst

@@ -808,6 +808,13 @@ Object Protocol
    that this function doesn't call into the Python interpreter. This function
    cannot fail.
 
+   .. caution::
+
+      For objects where :c:expr:`Py_REFCNT(op) == 1` is always true this
+      function will return false when checked in a different thread than the
+      allocation. This can lead to subtle behavior change bugs between the
+      free-threaded and GIL-enabled builds (:gh:`156995`).
+
    .. versionadded:: 3.14
 
 .. c:function:: int PyUnstable_SetImmortal(PyObject *op)