Doc: Document QTaskInterface::done() signal

Add documentation for the missing signal; fixes documentation
warning:

    (qdoc) warning: No documentation for signal
    'QtTaskTree::QTaskInterface::done(QtTaskTree::DoneResult result)'

Pick-to: 6.12 6.11
Change-Id: I8592baca5d5058bdffe4c45a27359ceb0d9bcf61
Reviewed-by: Topi Reinio <topi.reinio@qt.io>