Fix QTextStream examples and documentation to use Qt:: modifiers

Qt 5.15 deprecates the QTextStream manipulators that are in the global C++ namespace. However, this is not reflected everywhere in the documentation:

• Example code use the modifiers without Qt:: prefix
• The documentation for the modifiers like https://doc-snapshots.qt.io/qt5-5.15/qtextstream.html#oct also do not mention it should be called from Qt:: namespace