Bug 1724645 - document StructuredSerializeOptions. r=smaug

The web standard linked from MessagePort.webidl doesn't make it quickly clear
what StructuredSerializeOptions is used for so I added a brief summary to make
it quicker to figure out. Note: I don't have much experience with this class so
my description may be incomplete.

Differential Revision: https://phabricator.services.mozilla.com/D142624

dom/webidl/MessagePort.webidl

@@ -22,6 +22,7 @@ interface MessagePort : EventTarget {
   attribute EventHandler onmessageerror;
 };
 
+// Used to declare which objects should be transferred.
 dictionary StructuredSerializeOptions {
   sequence<object> transfer = [];
 };