QSharedPointer doc needs to deal with forward declaration issue

QScopedPointer and and QSharedPointer have interesting, non-intuitive issues with forward-declared classes when being used as class members. The requirements to use them correctly in such contexts are documented for QScopedPointer (paragraph "Forward Declared Pointers"), but not for QSharedPointer. In my experience, developers stumble over this issue all the time, so it should definitely be explained in the documentation.