Document what QWidget::winId() returns on Mac OS X.
Document what QWidget::winId() returns on Mac OS X.

There are different types depending on Carbon and Cocoa, and it is
probably helpful to point that out.

Task-number: 251001
Reviewed-by: Kavindra

igor-sfdc/qt-wk,RLovelett/qt,RLovelett/qt,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,KDE/qt,igor-sfdc/qt-wk,radekp/qt,igor-sfdc/qt-wk,RLovelett/qt,KDE/qt,pruiz/wkhtmltopdf-qt,radekp/qt,RLovelett/qt,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,KDE/qt,pruiz/wkhtmltopdf-qt,KDE/qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,RLovelett/qt,KDE/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt,KDE/qt,RLovelett/qt,KDE/qt,igor-sfdc/qt-wk,RLovelett/qt,radekp/qt,pruiz/wkhtmltopdf-qt,KDE/qt,igor-sfdc/qt-wk,RLovelett/qt,radekp/qt,igor-sfdc/qt-wk,pruiz/wkhtmltopdf-qt,radekp/qt,pruiz/wkhtmltopdf-qt,pruiz/wkhtmltopdf-qt