QSaveFile: mention that you need to setFileName() before open()

... in the descriptiong of the (QObject*) ctor.

Pick-to: 6.8 6.5 5.15
Change-Id: I1b8a788eff1fbb50134b51de7d9ee8f7cb2af785
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 6397dac97962e40104e6a94bc147e093af8186e2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/io/qsavefile.cpp

@@ -90,6 +90,7 @@ QSaveFile::QSaveFile(const QString &name)
 
 /*!
     Constructs a new file object with the given \a parent.
+    You need to call setFileName() before open().
 */
 QSaveFile::QSaveFile(QObject *parent)
     : QFileDevice(*new QSaveFilePrivate, parent)