add docs

Author: kumaraditya303

docs/api.rst

@@ -70,6 +70,10 @@ Python 3.15
 
    On PyPy, always returns ``-1``.
 
+.. c:function:: int PyUnstable_SetImmortal(PyObject *op)
+
+   See `PyUnstable_SetImmortal() documentation  <https://docs.python.org/dev/c-api/object.html#c.PyUnstable_SetImmortal>`__.
+
 
 Python 3.14
 -----------

docs/changelog.rst

@@ -1,6 +1,10 @@
 Changelog
 =========
 
+* 2026-02-12: Add functions:
+
+  * ``PyUnstable_SetImmortal()``
+
 * 2025-10-14: Add functions:
 
   * ``PyTuple_FromArray()``

pythoncapi_compat.h

@@ -2660,6 +2660,10 @@ PyUnstable_Unicode_GET_CACHED_HASH(PyObject *op)
 #endif
 
 #if 0x030E0000 <= PY_VERSION_HEX && PY_VERSION_HEX < 0x030F00A7 && !defined(PYPY_VERSION)
+// Immortal objects were implemented in Python 3.12, however there is no easy API
+// to make objects immortal until 3.14 which has _Py_SetImmortal(). Since
+// immortal objects are primarily needed for free-threading, this API is implemented
+// for 3.14 and above.
 extern void _Py_SetImmortal(PyObject *op);
 static inline int
 PyUnstable_SetImmortal(PyObject *op)