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>

src/concurrent/qtconcurrentrun.cpp

@@ -259,9 +259,8 @@
     \since 5.4
     \fn QFuture<T> QtConcurrent::run(QThreadPool *pool, Function function, ...);
 
-    Runs \a function in a separate thread. The thread is taken from the
+    Schedules \a function on \a pool. Note that \a function may not run
-    QThreadPool \a pool. Note that \a function may not run immediately; \a function
+    immediately; \a function will only be run once a thread becomes available.
-    will only be run once a thread becomes available.
 
     \include qtconcurrentrun.cpp run-description
 */

src/corelib/thread/qfuture.qdoc

@@ -122,6 +122,12 @@
     QtFuture::makeReadyValueFuture(), QtFuture::makeReadyRangeFuture(), and
     QtFuture::makeExceptionalFuture().
 
+    \note Some APIs (see \l {QFuture::then()} or various QtConcurrent method
+    overloads) allow scheduling the computation to a specific thread pool.
+    However, QFuture implements a work-stealing algorithm to prevent deadlocks
+    and optimize thread usage. As a result, computations can be executed
+    directly in the thread which requests the QFuture's result.
+
     \note To start a computation and store results in a QFuture, use QPromise or
     one of the APIs in the \l {Qt Concurrent} framework.
 
@@ -1257,8 +1263,7 @@
 
     Attaches a continuation to this future, allowing to chain multiple asynchronous
     computations if desired. When the asynchronous computation represented by this
-    future finishes, \a function will be invoked in a separate thread taken from the
+    future finishes, \a function will be scheduled on \a pool.
-    QThreadPool \a pool.
 
     \sa onFailed(), onCanceled()
 */