Document Qt version for list of third party attributions

The list of third-party code attributions in

http://doc.qt.io/qt-5/licenses-used-in-qt.html

and in the documentation of the various modules, e.g. in

http://doc.qt.io/qt-5/qtcore-index.html#licenses-and-attributions

are only valid for the specific Qt patch level release that the documentation is generated from. This should be explicitly stated in the documentation.