Update use cases section with iProov's.

Try to describe iProov's use case in a more general form: using the Ambient
Light Sensor API to help determine if a given video stream can be used for
their purposes or not.

While here, remove the use case about hand user gestures in gaming; that
looks too far-fetched at this point and best served by XR and
Accelerometer/Gyroscope APIs than ALS.

Author: Raphael Kubo da Costa

index.bs

@@ -289,8 +289,9 @@ Use Cases and Requirements {#usecases-requirements}
 - A Web application provides input for a smart home system to control lighting.
 - A Web aplication checks whether light level at work space is sufficient.
 - A Web application calculates settings for a camera with manual controls (aperture, shutter speed, ISO).
-- A Web application monitors light level changes produced by hovering hand user gesture and
-  interprets them to control a game character.
+- A Web application checks the current light level to determine whether a
+  camera stream will contain data that is accurate enough for its purposes
+  (e.g. human presence verification).
 
 While some of the use cases may benefit from obtaining precise ambient light measurements, the use
 cases that convert ambient light level fluctuations to user input events would benefit from