createObjectIn: missing document

I came across "createObjectIn", which is still a `function` when tested in content script, but couldn’t find any reference to it on MDN.

It became available with [exportFunction](https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts#exportfunction) and [cloneinto](https://developer.mozilla.org/docs/Mozilla/Add-ons/WebExtensions/Sharing_objects_with_page_scripts#cloneinto).

- [Changes to unsafeWindow for the Add-on SDK](https://blog.mozilla.org/addons/2014/04/10/changes-to-unsafewindow-for-the-add-on-sdk/)
- [Greasemonkey environment is missing cloneInto, createObjectIn and exportFunction](https://github.com/sindresorhus/globals/issues/146)
- [930523 - createObjectIn should waive before returning](https://bugzilla.mozilla.org/show_bug.cgi?id=930523)

https://github.com/mdn/content/blob/590795c0f1d1b54f17dcab9d66555c639c37f051/files/en-us/mozilla/firefox/releases/8/index.md?plain=1#L98