Fix documentation of Q_NAMESPACE

We need to add the \relates command otherwise it won't appear in the documentation

Change-Id: I134776c1528445761a7539cf687e4855d39eb7a7
Reviewed-by: BogDan Vatra <bogdan@kdab.com>

src/corelib/kernel/qobject.cpp

@@ -4429,6 +4429,7 @@ QDebug operator<<(QDebug dbg, const QObject *o)
 
 /*!
     \macro Q_NAMESPACE
+    \relates QObject
     \since 5.8
 
     The Q_NAMESPACE macro can be used to add QMetaObject capabilities