platform/x86: dell-wmi: Add a better description for "stealth mode"

This is based on Mario's explanation and observation of my laptop.

Suggested-by: "Pali Rohár" <pali.rohar@gmail.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Mario Limonciello <mario_limonciello@dell.com>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>