Add part of documentation

Author: kulikjak

Doc/library/os.rst

@@ -4611,11 +4611,18 @@ written in Python, such as a mail server's external command delivery program.
 
       Performs ``os.closerange(fd, INF)``.
 
+   .. data:: POSIX_SPAWN_CHDIR
+
+      (``os.POSIX_SPAWN_CHDIR``, *path*)
+
+      Performs ``os.chdir(path)``.
+
    These tuples correspond to the C library
    :c:func:`!posix_spawn_file_actions_addopen`,
    :c:func:`!posix_spawn_file_actions_addclose`,
-   :c:func:`!posix_spawn_file_actions_adddup2`, and
-   :c:func:`!posix_spawn_file_actions_addclosefrom_np` API calls used to prepare
+   :c:func:`!posix_spawn_file_actions_adddup2`,
+   :c:func:`!posix_spawn_file_actions_addclosefrom_np`, and
+   :c:func:`!posix_spawn_file_actions_addchdir_np` API calls used to prepare
    for the :c:func:`!posix_spawn` call itself.
 
    The *setpgroup* argument will set the process group of the child to the value
@@ -4661,8 +4668,9 @@ written in Python, such as a mail server's external command delivery program.
       *env* parameter accepts ``None``.
 
    .. versionchanged:: 3.13
-      ``os.POSIX_SPAWN_CLOSEFROM`` is available on platforms where
-      :c:func:`!posix_spawn_file_actions_addclosefrom_np` exists.
+      ``os.POSIX_SPAWN_CLOSEFROM`` and ``os.POSIX_SPAWN_CHDIR`` are available
+      on platforms where :c:func:`!posix_spawn_file_actions_addclosefrom_np`
+      and :c:func:`!posix_spawn_file_actions_addchdir_np` exist.
 
    .. availability:: Unix, not Emscripten, not WASI.