Mention in the DNR docs that ||* is not allowed in a URL pattern

Bug: 696822
Change-Id: I5dff25148585cbe97a6dae5569f099679e52f618
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1919827Reviewed-by: Karan Bhatia <karandeepb@chromium.org>
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Auto-Submit: Theodore Dubois <tbodt@google.com>
Cr-Commit-Position: refs/heads/master@{#715982}

extensions/common/api/declarative_net_request.idl

@@ -134,6 +134,9 @@ namespace declarativeNetRequest {
     //
     // If omitted, all urls are matched. An empty string is not allowed.
     //
+    // A pattern beginning with <code>||*</code> is not allowed. Use
+    // <code>*</code> instead.
+    //
     // Note: The <code>urlFilter</code> must be composed of only ASCII
     // characters. This is matched against a url where the host is encoded in
     // the punycode format (in case of internationalized domains) and any other