Doc: Use Qml DefaultProperty as example for Q_CLASSINFO

Setting a DefaultProperty is arguably something
more useful to people than the 'made up' version example.

Also add more links to the relevant QML pages.

Pick-to: 6.6
Change-Id: Id967df7ddf81511299f95b9fd24f56af671fb855
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
(cherry picked from commit 98ba0d4e95353febd5a4b14fe2eb9cbd4259bf0a)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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

@@ -96,7 +96,7 @@ object->setProperty("priority", "VeryHigh");
 
 
 //! [7]
-Q_CLASSINFO("Version", "3.0.0")
+Q_CLASSINFO("DefaultProperty", "content")
 //! [7]
 
 //! [8]

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

@@ -277,7 +277,9 @@
 
     Connected to the property system is an additional macro,
     Q_CLASSINFO(), that can be used to attach additional
-    \e{name}--\e{value} pairs to a class's meta-object, for example:
+    \e{name}--\e{value} pairs to a class's meta-object. This is
+    used for instance to mark a property as the \e default one
+    in the context of \l{QML Object Types}:
 
     \snippet code/doc_src_properties.cpp 7
 
@@ -299,5 +301,5 @@
     and the general tips on implementing and using
     \l {Qt Bindable Properties}{bindable properties}.
 
-    \sa {Qt Bindable Properties}
+    \sa {Qt Bindable Properties}, {Defining QML Types from C++}
 */

src/corelib/kernel/qobject.cpp

@@ -4417,19 +4417,22 @@ QDebug operator<<(QDebug dbg, const QObject *o)
     \relates QObject
 
     This macro associates extra information to the class, which is available
-    using QObject::metaObject(). Qt makes only limited use of this feature in
-    \l{Qt D-Bus} and \l{Qt Qml} modules.
-
-    The extra information takes the form of a \a Name string and a \a Value
-    literal string.
+    using QObject::metaObject(). The extra information takes the form of a
+    \a Name string and a \a Value literal string.
 
     Example:
 
     \snippet code/src_corelib_kernel_qobject.cpp 35
 
+    Qt makes use of the macro in \l{Qt D-Bus} and \l{Qt Qml} modules.
+    For instance, when defining \l{QML Object Types} in C++, you can
+    designate a property as the \e default one:
+
+    \snippet code/doc_src_properties.cpp 7
+
     \sa QMetaObject::classInfo()
     \sa {Using Qt D-Bus Adaptors}
-    \sa {Extending QML}
+    \sa {Defining QML Types from C++}
 */
 
 /*!