tst_QFutureSynchronizer: describe how setFutureAliasingExistingMember() works

So no-one 'fixes' the test by pinning synchronizer.futures() into a
named variable or collapsing the two lines into one. Both would break
the premiss of the test.

Amends e8dcbaaaf61ee2164db61e70a5d61d7a5b849371.

Change-Id: I7a98c382a7db0bf3ff369dcaf61af0942796d6a3
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit 70a7a695fdbaccd042aa4371cbb231ced2d9b499)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/auto/corelib/thread/qfuturesynchronizer/tst_qfuturesynchronizer.cpp

@@ -45,6 +45,11 @@ void tst_QFutureSynchronizer::setFutureAliasingExistingMember()
     // WHEN: calling setFuture() with an alias of the QFuture already in `synchronizer`:
     //
     for (int i = 0; i < 2; ++i) {
+        // The next line triggers -Wdangling-reference, but it's a FP because
+        // of implicit sharing. We cannot keep a copy of synchronizer.futures()
+        // around to avoid the warning, as the extra copy would cause a detach()
+        // of m_futures inside setFuture() with the consequence that `f` no longer
+        // aliases an element in m_futures, which is the goal of this test.
         const auto &f = synchronizer.futures().constFirst();
         synchronizer.setFuture(f);
     }