QStringList: add missing API docs

Split from QTBUG-116859

QStringList::indexOf/lastIndexOf({const QString &, QStringView}) have no API docs in QStringList. Given that it gets them by inheritance from QListSpecialMethodsBase<QString> and QList but they should sill be mentioned in the API docs to make them more discoverable for new users (a project is always judged by its API docs).

The commits adding these API docs should be backported to current LTS (Qt 6.5).