Clarify that the open audit event raised by os.open() always supplies None for its mode field. Access modes are available through flags, while the file creation permission bits passed as the function's mode parameter are not included in the event.

Fixes #158916.

Validation:

• Captured audit events on the current CPython debug build for builtin binary open(), read-only os.open(), and os.open() with file creation permissions; checked the mode and access flags.
• Full HTML documentation build with warnings treated as errors.
• Sphinx lint and patchcheck.

• Issue: "open" audit event "mode" does not contain information for os.open calls #158916