ios: Document WebThread task posting order guarantees.

Replicating the equivalent BrowserThread documentation for ios, see
https://chromium-review.googlesource.com/c/chromium/src/+/1246361

Bug: 878356, 882872
Change-Id: Ie9470baecbce1dbb5b6c45852836db238cffe3b7
Reviewed-on: https://chromium-review.googlesource.com/1253763Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/master@{#595418}

ios/web/public/web_task_traits.h

@@ -31,6 +31,10 @@ struct NonNestable {};
 // To obtain a TaskRunner for the UI thread (analogous for the IO thread):
 //     base::CreateSingleThreadTaskRunnerWithTraits({WebThread::UI});
 //
+// Tasks posted to the same WebThread with the same traits will be executed
+// in the order they were posted, regardless of the TaskRunners they were
+// posted via.
+//
 // See //base/task/post_task.h for more detailed documentation.
 //
 // Posting to a WebThread must only be done after it was initialized (ref.