gh-142302: Fix mkstemp() documentation: clarify file descriptor inheritance behavior

Doc/library/os.rst

@@ -1969,7 +1969,11 @@ can be inherited by child processes.  Since Python 3.4, file descriptors
 created by Python are non-inheritable by default.
 
 On UNIX, non-inheritable file descriptors are closed in child processes at the
-execution of a new program, other file descriptors are inherited.
+execution of a new program (via :func:`os.execl` and related functions), but
+they remain accessible after :func:`os.fork` until an exec call occurs. In other
+words, a forked child process can still use the file descriptor, but it will be
+closed if that child process calls exec to run a new program. Inheritable file
+descriptors are inherited across both fork and exec calls.
 
 On Windows, non-inheritable handles and file descriptors are closed in child
 processes, except for standard streams (file descriptors 0, 1 and 2: stdin, stdout

Doc/library/tempfile.rst

@@ -225,8 +225,12 @@ The module defines the following user-callable items:
    properly implements the :const:`os.O_EXCL` flag for :func:`os.open`.  The
    file is readable and writable only by the creating user ID.  If the
    platform uses permission bits to indicate whether a file is executable,
-   the file is executable by no one.  The file descriptor is not inherited
-   by child processes.
+   the file is executable by no one.
+
+   The file descriptor is not inherited by child processes across
+   :func:`exec <os.execl>` calls, but will be inherited by child
+   processes created via :func:`os.fork`. See :ref:`fd_inheritance`
+   for more information.
 
    Unlike :func:`TemporaryFile`, the user of :func:`mkstemp` is responsible
    for deleting the temporary file when done with it.