docs: Mention difference in executable bits of created files with open(…, opener=…)

Default `opener` value differs, leading to subtle bugs if users read the current text as “I can just use `os.open` as drop-in”.

Author: ntninja

Doc/library/functions.rst

@@ -1494,7 +1494,8 @@ are always available.  They are listed here in alphabetical order.
    file descriptor for the file object is then obtained by calling *opener* with
    (*file*, *flags*). *opener* must return an open file descriptor (passing
    :mod:`os.open` as *opener* results in functionality similar to passing
-   ``None``).
+   ``None`` but will also request, subject to the process ``umask``, the file be
+   created with executable permissions on Unix).
 
    The newly created file is :ref:`non-inheritable <fd_inheritance>`.