Update comments for preferences that are now policy managed

Per feedback from:

https://chromium-review.googlesource.com/c/chromium/src/+/1465070

updated documentation for preferences that can be overridden by
administrator policies.

Bug: b:124113875
Change-Id: I2b47ae6a031b3e65dcbc1348d4050600c63a8307
Reviewed-on: https://chromium-review.googlesource.com/c/1483258Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Eyor Alemayehu <eyor@google.com>
Cr-Commit-Position: refs/heads/master@{#635116}

components/arc/arc_prefs.cc

@@ -111,7 +111,8 @@ const char kEngagementTimeOsVersion[] =
 const char kEngagementTimeDayId[] = "arc.metrics.engagement_time.day_id";
 // A preference that indicates the user has allowed voice interaction services
 // to access the "context" (text and graphic content that is currently on
-// screen).
+// screen). This preference can be overridden by the
+// VoiceInteractionContextEnabled administrator policy.
 const char kVoiceInteractionContextEnabled[] =
     "settings.voice_interaction.context.enabled";
 // A preference that indicates the user has enabled voice interaction services.
@@ -121,7 +122,8 @@ const char kVoiceInteractionEnabled[] = "settings.voice_interaction.enabled";
 const char kVoiceInteractionHotwordAlwaysOn[] =
     "settings.voice_interaction.hotword.always_on";
 // A preference that indicates the user has allowed voice interaction services
-// to use hotword listening.
+// to use hotword listening. This preference can be overridden by the
+// VoiceInteractionHotwordEnabled administrator policy.
 const char kVoiceInteractionHotwordEnabled[] =
     "settings.voice_interaction.hotword.enabled";
 // A preference that indicates whether microphone should be open when the voice