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>

include/acpi/video.h

@@ -32,6 +32,10 @@ extern int acpi_video_get_edid(struct acpi_device *device, int type,
 			       int device_id, void **edid);
 extern enum acpi_backlight_type acpi_video_get_backlight_type(void);
 extern void acpi_video_set_dmi_backlight_type(enum acpi_backlight_type type);
+/*
+ * Note: The value returned by acpi_video_handles_brightness_key_presses()
+ * may change over time and should not be cached.
+ */
 extern bool acpi_video_handles_brightness_key_presses(void);
 #else
 static inline int acpi_video_register(void) { return 0; }