QBitmap::fromPixmap: document \since

Change-Id: Ia8b4e20dc98e3b184b6e2ba9844907f813dbc1ab
Pick-to: 6.2 6.3
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/gui/image/qbitmap.cpp

@@ -260,6 +260,8 @@ QBitmap QBitmap::fromData(const QSize &size, const uchar *bits, QImage::Format m
     If the pixmap has a depth greater than 1, the resulting bitmap
     will be dithered automatically.
 
+    \since 6.0
+
     \sa QPixmap::depth()
 */