doc: add qmllint incompatible-type warnings (for 6.8)

Add description of the warning introduced in 6.8 and an example on
how to fix it.

No need to pick back to 6.7 as this warning was introduced in 6.8 via
9cdd485c2b.

Pick-to: 6.8
Task-number: QTBUG-118112
Change-Id: Ice2a161cb195bc47dff2aa98bfee882d2ee6d417
Reviewed-by: Semih Yavuz <semih.yavuz@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

src/qml/doc/src/qmllint/incompatible-type.qdoc

@@ -95,6 +95,34 @@ Item {
 
 \endqml
 
+\section1 Construction from string is deprecated; Use structured value type construction instead
+
+\section2 What happened?
+You constructed a \l{qqmlengine.html#QML_STRUCTURED_VALUE}{structured value type} using a string.
+
+\section2 Why is this bad?
+This is deprecated and prone to typos.
+
+\section2 Example
+\qml
+import QtQuick
+
+Item {
+    property point p: "5, 6"
+}
+
+\endqml
+To fix this warning, populate the structured value type as explained
+\l{qqmlengine.html#QML_STRUCTURED_VALUE}{here} instead of binding a string to the property:
+\qml
+import QtQuick
+
+Item {
+    property point p: ({ x: 5, y: 6 })
+}
+
+\endqml
+
 \section1 Function without return type annotation returns
 
 \section2 What happened?