Fix documentation about QStringLiteral

The fallback for QStringLiteral in case C++11 features are not enabled
is QString::fromUtf8(), not QLatin1String().

Also, the result of a QStringLiteral expression _is_ a QString.

Change-Id: Ib9c2f4c13fff237de3acb2e0f64027bacea6271c
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>