gh-142302: Fix mkstemp() documentation: clarify file descriptor inheritance behavior (#142338)

The documentation incorrectly stated that the file descriptor is not
inherited by child processes. In reality, the close-on-exec flag (when
available) only prevents inheritance across exec() calls, not fork().

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: raixyzaditya

Doc/library/os.rst

@@ -1979,7 +1979,8 @@ 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, other file descriptors are inherited. Note that
+non-inheritable file descriptors are still *inherited* by child processes on :func:`os.fork`.
 
 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 +225,9 @@ 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 :ref:`not inherited by child processes <fd_inheritance>`.
 
    Unlike :func:`TemporaryFile`, the user of :func:`mkstemp` is responsible
    for deleting the temporary file when done with it.