gh-127896: Add missing document of PySequence_In

Doc/c-api/sequence.rst

@@ -105,6 +105,15 @@ Sequence Protocol
    equivalent to the Python expression ``value in o``.
 
 
+.. c:function:: int PySequence_In(PyObject *o, PyObject *value)
+
+   Alias for :c:func:`PySequence_Contains`.
+
+   .. deprecated:: 3.14
+      The function is :term:`soft deprecated` and should no longer be used to
+      write new code.
+
+
 .. c:function:: Py_ssize_t PySequence_Index(PyObject *o, PyObject *value)
 
    Return the first index *i* for which ``o[i] == value``.  On error, return

Doc/whatsnew/3.14.rst

@@ -1073,6 +1073,10 @@ Deprecated
   :c:macro:`!isfinite` available from :file:`math.h`
   since C99.  (Contributed by Sergey B Kirpichev in :gh:`119613`.)
 
+* The previously undocumented function :c:func:`PySequence_In` is :term:`soft deprecated`.
+  Use :c:func:`PySequence_Contains` instead.
+  (Contributed by Yuki Kobayashi in :gh:`127896`.)
+
 .. Add C API deprecations above alphabetically, not here at the end.
 
 .. include:: ../deprecations/c-api-pending-removal-in-3.15.rst

Misc/NEWS.d/next/C_API/2024-12-16-07-12-15.gh-issue-127896.HmI9pk.rst

@@ -0,0 +1,2 @@
+The previously undocumented function :c:func:`PySequence_In` is :term:`soft deprecated`.
+Use :c:func:`PySequence_Contains` instead.