Doc: complete the snippet for a dialog with extension

Add the setting of the layout's size constraint to 'fixed' back to the
snippet, so that the dialog automatically resizes when the extension
gets hidden. The user won't be able to resize the dialog, but allowing
the user to resize the dialog and to show and hide the extension anyway
leads to unpredictable results for the user.

Amends 11da92ba94570e5eec01597fe09f0a9a48acc677.

Pick-to: 6.6
Change-Id: Ie74ca36eaa1a8e9567e0d5826f91f8633e5cbc05
Reviewed-by: David Faure <david.faure@kdab.com>
(cherry picked from commit 51a1dfe72ddd922cd6caf28d532b3311db767989)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/dialogs/qdialog.cpp

@@ -324,6 +324,10 @@ QVariant QDialogPrivate::styleHint(QPlatformDialogHelper::StyleHint hint) const
 
     \snippet dialogs/dialogs.cpp extension
 
+    By setting the \l{QLayout::}{sizeConstraint} property of the dialog's
+    layout to \l{QLayout::}{SetFixedSize}, the dialog will not be resizable
+    by the user, and will automatically shrink when the extension gets hidden.
+
     \sa QDialogButtonBox, QTabWidget, QWidget, QProgressDialog,
         {Standard Dialogs Example}
 */

src/widgets/doc/snippets/dialogs/dialogs.cpp

@@ -236,6 +236,8 @@ void extension()
     QVBoxLayout *mainLayout;
 
 //! [extension]
+    mainLayout->setSizeConstraint(QLayout::SetFixedSize);
+
     findButton = new QPushButton(tr("&Find"));
     moreButton = new QPushButton(tr("&More..."));
     moreButton->setCheckable(true);