Clarify |stopSyncAndClearIdentity| with additional comment.

Namely, it is OK to call it multiple times.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I7348628de6e39ca50461c25db7343cd46b5f069a
Reviewed-on: https://chromium-review.googlesource.com/c/1278061Reviewed-by: Hiroshi Ichikawa <ichikawa@chromium.org>
Commit-Queue: Hiroshi Ichikawa <ichikawa@chromium.org>
Cr-Commit-Position: refs/heads/master@{#599124}

ios/web_view/public/cwv_sync_controller.h

@@ -70,7 +70,7 @@ CWV_EXPORT
                    dataSource:
                        (__weak id<CWVSyncControllerDataSource>)dataSource;
 
-// Stops syncs and nils out |currentIdentity|.
+// Stops syncs and nils out |currentIdentity|. This method is idempotent.
 - (void)stopSyncAndClearIdentity;
 
 // If |passphraseNeeded| is |YES|. Call this to unlock the sync data.