Clarify documentation for the QMessageBox QML type

The documentation for the QMessageBox qml type needs to be updated to address the following:

1. There are no description/link of "identifier" parameters under "Method Documentation" such as:

>Button critical(string identifier,

There is description what "identifier" is here actually but it is not easy to find this. http://doc.qt.io/qtinstallerframework/scripting-qmessagebox.html#scripted-installations

And it should be described what default value or optional also.

2. It should state that "buttons" is a list of buttons presented on the message dialog.

>Button critical(string identifier, string title, string text, Buttons buttons = QMessageBox.Ok, 

3. It should state that "button" is a button that will be selected by default.

>Button critical(string identifier, string title, string text, Buttons buttons = QMessageBox.Ok, Button button = QMessageBox.NoButton