Docs: fix warning from old QCheckBox::checkStateChanged overload

The overload carrying the new state as an integer is deprecated, but
still documented, so we need to document the parameter as well. Bring
the full documentation back to silence the warning.

Amends 5a96d13bb5abd5339cf21dd1de7a17152c71f0fc.

Change-Id: I5253e68e4ad7b42fc552feb958cc0ac40d685609
Reviewed-by: Safiyyah Moosa <safiyyah.moosa@qt.io>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit 31de58990661f6fdc9f1145e16745916d1c6df59)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qcheckbox.cpp

@@ -93,6 +93,11 @@ public:
     \fn void QCheckBox::stateChanged(int state)
 
     \deprecated [6.9] Use checkStateChanged(Qt::CheckState) instead.
+
+    This signal is emitted whenever the checkbox's state changes, i.e.,
+    whenever the user checks or unchecks it.
+
+    \a state contains the checkbox's new Qt::CheckState.
 */
 
 /*!