x-doc-messaging.json: MDN says min 6.0 for objects

Update "Cross-document messaging" (window.postMessage) to state that Firefox 6.0 is required for non-strings as data.

https://developer.mozilla.org/en-US/docs/DOM/window.postMessage:

Prior to Gecko 6.0 (Firefox 6.0 / Thunderbird 6.0 / SeaMonkey 2.3), the data parameter must be a string. Starting in Gecko 6.0 (Firefox 6.0 / Thunderbird 6.0 / SeaMonkey 2.3), the data parameter is serialized using the structured clone algorithm.