Clarify fd inheritance behavior for fork vs exec

Update mkstemp() documentation to clarify that file descriptors are
not inherited across exec() calls but remain accessible after fork().
Add reference link to the fd_inheritance section.

Also update the fd_inheritance section in os.rst to explicitly
explain the distinction between fork() and exec() behavior for
non-inheritable file descriptors.

Addresses feedback from vstinner.

Author: raixyzaditya

Doc/library/tempfile.rst

@@ -230,9 +230,8 @@ The module defines the following user-callable items:
    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 
-   `File Descriptor Inheritance <https://docs.python.org/dev/library/os.html#fd-inheritance>`_
-   for more information.
-   
+   :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.