Improve Screen_brightness documentation

- document screen_brightness, such that it is clear that the range is
  from 1 to 255.
- document screen_brightness_mode_automatic, such that it is clear this
  affects screen_brightness

Bug: 331750107
Flag: EXEMPT documentation fixes only
Test: builds

Change-Id: I9e41edbbb3c0163f6a2529d88ff6947e9fbf74b2
1 file changed