[On-device adaptive brightness] Update flag description

This description makes it clearer that the flag is only used to control
the ML model if it exists.

AboutFlagsHistogramTest*

Test: unit_tests --gtest_filter=AboutFlagsTest*,
Bug: 881215
Change-Id: Ia226f98d954ad96c680c593716509796b17d0aa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2346098
Commit-Queue: Jia Meng <jiameng@chromium.org>
Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#796667}

chrome/browser/flag_descriptions.cc

@@ -330,8 +330,10 @@ const char kAutofillUseImprovedLabelDisambiguationDescription[] =
 
 const char kAutoScreenBrightnessName[] = "Auto Screen Brightness model";
 const char kAutoScreenBrightnessDescription[] =
-    "Uses Auto Screen Brightness model to adjust screen brightness based on "
-    "ambient light";
+    "Uses Auto Screen Brightness ML model (if it exists) to adjust screen "
+    "brightness based on ambient light. If disabled, screen brightness "
+    "will be controlled by the heuristic method from power (and only on "
+    "devices that have ambient light sensors).";
 
 const char kAvatarToolbarButtonName[] = "Avatar Toolbar Button";
 const char kAvatarToolbarButtonDescription[] =