QBuffer: test and document open() behavior

QBuffer::open() was only documented as \reimp, so its behavior
regarding WriteOnly was never actually described.

Add a test and document the outcome.

Change-Id: I75c49cd3f6a1961bcaece4a92a4e479bb3300d36
Reviewed-by: Mate Barany <mate.barany@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit c262a1805a062fd99abc890f58ea951da80ff0af)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qbuffer.cpp

@@ -277,6 +277,10 @@ void QBuffer::setData(const QByteArray &data)
 
 /*!
    \reimp
+
+   Unlike QFile, opening a QBuffer QIODevice::WriteOnly does not truncate it.
+   However, pos() is set to 0. Use QIODevice::Append or QIODevice::Truncate to
+   change either behavior.
 */
 bool QBuffer::open(OpenMode flags)
 {

tests/auto/corelib/io/qbuffer/tst_qbuffer.cpp

@@ -15,6 +15,7 @@ class tst_QBuffer : public QObject
     Q_OBJECT
 private slots:
     void open();
+    void openWriteOnlyDoesNotTruncate();
     void getSetCheck();
     void readBlock();
     void readBlockPastEnd();
@@ -111,6 +112,29 @@ void tst_QBuffer::open()
     b.close();
 }
 
+void tst_QBuffer::openWriteOnlyDoesNotTruncate()
+{
+    QBuffer b;
+    const auto data = QByteArrayLiteral("Hey, presto!");
+
+    {
+        QVERIFY(b.open(QIODevice::WriteOnly));
+        b.write(data);
+        b.close();
+    }
+    {
+        QVERIFY(b.open(QIODevice::ReadOnly));
+        QCOMPARE(b.readAll(), data);
+        b.close();
+    }
+    {
+        QVERIFY(b.open(QIODevice::WriteOnly));
+        QCOMPARE(b.size(), data.size());
+        QCOMPARE(b.pos(), 0);
+        b.close();
+    }
+}
+
 // some status() tests, too
 void tst_QBuffer::readBlock()
 {