Document best practices for working with string literals

https://doc.qt.io/qt-6/qstring.html has some information about QString usage, but doesn't mention literals from what I can see.

• https://en.cppreference.com/w/cpp/language/string_literal
• https://en.cppreference.com/w/cpp/language/user_literal

We have e.g. R("foo"), u"foo", u"foo"_qs, QString(u"foo"), etc, etc.