ACPI / video: Document acpi_video_handles_brightness_key_presses() a bit

Document that acpi_video_handles_brightness_key_presses()'s return value
may change over time and should not be cached.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>