Clarify mkstemp() fd inheritance is specific to exec() calls

The original documentation stated 'The file descriptor is not inherited
by child processes' without clarifying that this only applies to exec()
calls due to O_CLOEXEC (set via PEP 446). The fd is still inherited
and usable after fork() before exec().

Update documentation to specifically mention exec() calls for accuracy

Author: raixyzaditya

Doc/library/tempfile.rst

@@ -227,7 +227,8 @@ The module defines the following user-callable items:
    platform uses permission bits to indicate whether a file is executable,
    the file is executable by no one.
 
-   The file descriptor will be inherited by child processes.
+   The file descriptor is not inherited by child processes across
+   :func:`exec <os.execl>` calls.
 
    Unlike :func:`TemporaryFile`, the user of :func:`mkstemp` is responsible
    for deleting the temporary file when done with it.