thinkpad-acpi: R52 brightness_mode has been confirmed

We can stop pestering users for confirmation of the brightness_mode
default for firmware TP-76.

While at it, add a few missing comments in that quirk table.

Reported-by: Whoopie <whoopie79@gmx.net>
Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Cc: stable@kernel.org