Sync: Update comment on DISABLE_REASON_USER_CHOICE

The previous comment contained an obsolete TODO (which was resolved),
and missed some details

Bug: none
Change-Id: I217f74abd75504b32f80180c90121e3464182255
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1658016
Commit-Queue: Marc Treib <treib@chromium.org>
Auto-Submit: Marc Treib <treib@chromium.org>
Reviewed-by: Jan Krcal <jkrcal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#669160}

components/sync/driver/sync_service.h

@@ -126,13 +126,11 @@ class SyncService : public KeyedService {
     DISABLE_REASON_ENTERPRISE_POLICY = 1 << 1,
     // Sync can't start because there is no authenticated user.
     DISABLE_REASON_NOT_SIGNED_IN = 1 << 2,
-    // Sync is suppressed by user choice, either via platform-level toggle (e.g.
-    // Android's "ChromeSync" toggle), a “Reset Sync” operation from the
-    // dashboard on desktop/ChromeOS.
-    // NOTE: Other code paths that go through RequestStop also set this reason
-    // (e.g. disabling due to sign-out or policy), so it's only really
-    // meaningful when it's the *only* disable reason.
-    // TODO(crbug.com/839834): Only set this reason when it's meaningful.
+    // Sync is suppressed by user choice, either via the feature toggle in
+    // Chrome settings (which exists on Android and iOS), a platform-level
+    // toggle (e.g. Android's "ChromeSync" toggle), or a “Reset Sync” operation
+    // from the dashboard. This is also set if there's simply no signed-in user
+    // (in addition to DISABLE_REASON_NOT_SIGNED_IN).
     DISABLE_REASON_USER_CHOICE = 1 << 3,
     // Sync has encountered an unrecoverable error. It won't attempt to start
     // again until either the browser is restarted, or the user fully signs out