[macOS] improve the docs for standard menu items

This has already been discussed in a closed bug report QTBUG-17291, but it has been buried under all the other changes. Therefore, this new bug report.

The editing shortcuts (Cmd+X, Cmd+C, Cmd+V, Cmd+A) don't work in the native "Save As..." dialog started with QFileDialog::getSaveFileName(). The problem is reproducible only with the native dialog i.e. setting QFileDialog::DontUseNativeDialog can be used as a workaround. And note that context menu "Copy", "Paste", etc. work. As does the "Save As..." dialog in native applications.

This can be reproduced with the Standard Dialogs example (File Dialogs -> QFileDialog::getSaveFileName()) or even in Qt Creator File->Save As... dialog.