QFuture: mention the work-stealing algorithm in the docs

A number of users have explored the behavior and complained about it.
Lets add a \note explaining this. The function in charge is
QThreadPool::stealAndRunRunnable(QRunnable *), which is a private
function, exclusively used by:

  QFutureInterfaceBase::waitForResult()
  QFutureInterfaceBase::waitForFinished()

also update the documentation to reflect these changes accordingly.

Fixes: QTBUG-112351
Pick-to: 6.6 6.5 6.2 5.15
Change-Id: I839858cb449063d8af9bef64d2f35a6816a419b0
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

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