[3.11] gh-96288: Add a sentence to os.mkdir's docstring. (GH-96271). (#98065)

(cherry picked from commit 1523c9e)

Co-authored-by: Hagai Helman Tov <hagai.helman@gmail.com>

Author: JelleZijlstra

Modules/clinic/posixmodule.c.h

@@ -4560,12 +4560,13 @@ If dir_fd is not None, it should be a file descriptor open to a directory,
 dir_fd may not be implemented on your platform.
   If it is unavailable, using it will raise a NotImplementedError.
 
-The mode argument is ignored on Windows.
+The mode argument is ignored on Windows. Where it is used, the current umask
+value is first masked out.
 [clinic start generated code]*/
 
 static PyObject *
 os_mkdir_impl(PyObject *module, path_t *path, int mode, int dir_fd)
-/*[clinic end generated code: output=a70446903abe821f input=e965f68377e9b1ce]*/
+/*[clinic end generated code: output=a70446903abe821f input=a61722e1576fab03]*/
 {
     int result;
 #ifdef HAVE_MKDIRAT