Doc: Document BINDABLE attribute for Q_PROPERTY macro

Fixes: QTBUG-87036
Change-Id: Ia6067d887b9b697dbcb8e02bce5a560a8ac5eaf3
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
(cherry picked from commit af7f7732cb52888c468b3e86a67e61b83fee11e2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/corelib/doc/snippets/code/doc_src_properties.cpp

@@ -59,6 +59,7 @@ Q_PROPERTY(type name
            [SCRIPTABLE bool]
            [STORED bool]
            [USER bool]
+           [BINDABLE bindableProperty]
            [CONSTANT]
            [FINAL]
            [REQUIRED])

src/corelib/doc/src/objectmodel/properties.qdoc

@@ -131,6 +131,12 @@
     editable property for (checkable) buttons. Note that QItemDelegate
     gets and sets a widget's \c USER property.
 
+    \li The \c {BINDABLE bindableProperty} attribute indicates that the
+    property supports bindings, and that it is possible to set and inspect
+    bindings to this property via the meta object system (QMetaProperty).
+    \c bindableProperty names a class member of type QBindable<T>, where T
+    is the property type. This attribute was introduced in Qt 6.0.
+
     \li The presence of the \c CONSTANT attribute indicates that the property
     value is constant.  For a given object instance, the READ method of a
     constant property must return the same value every time it is called.  This