QAbstractSpinBox: Add more documentation cross links

stepUp() and stepDown() already linked to stepBy(), so add the
reverse too.

keyPressEvent() talks about stepBy() too, so add it to the cross
reference.

Change-Id: I22c841821331eaed9607cfb2807dcf0e2886d952
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>
Reviewed-by: Luca Beldi <v.ronin@yahoo.it>
Reviewed-by: Samuel Gaist <samuel.gaist@edeltech.ch>
Reviewed-by: Mitch Curtis <mitch.curtis@qt.io>

src/widgets/widgets/qabstractspinbox.cpp

@@ -619,6 +619,8 @@ void QAbstractSpinBox::stepDown()
     function. Note that this function is called even if the resulting
     value will be outside the bounds of minimum and maximum. It's this
     function's job to handle these situations.
+
+    \sa stepUp(), stepDown(), keyPressEvent()
 */
 
 void QAbstractSpinBox::stepBy(int steps)
@@ -970,6 +972,8 @@ void QAbstractSpinBox::paintEvent(QPaintEvent *)
     \row \li Page down
          \li This will invoke stepBy(-10)
     \endtable
+
+    \sa stepBy()
 */