Fix documentation for qRegisterMetaType()

Docs for Q_DECLARE_METATYPE() says currently:

Note that if you intend to use the type in queued signal and slot connections or in QObject's property system, you also have to call qRegisterMetaType() since the names are resolved at runtime.

And for qRegisterMetaType():

To use the type T in queued signal and slot connections, qRegisterMetaType<T>() must be called before the first connection is established.

This is basically true when we are using string-based connections. It looks like with typed connections the call to qRegisterMetaType() isn't needed anymore (in contrary to Qt 5.15), even for queued connections, as no runtime names resolving is involved in this case.