Allows the user to make modifications to the document.
The completion handler block object passed in to be invoked after unlocking is completed, regardless of success or failure.
By default, this method invokes the unlock(completionHandler:) method to unlock the document. This method disables autosaving safety checking, meaning that checkAutosavingSafety() will no longer be invoked on this document. When unlocking succeeds, the isLocked method will begin returning false.