Docs: add QT_PROTO_INCLUDES note for ProtobufQt{Core,Gui}Types

This was missing before and is a crucial information.

Pick-to: 6.9 6.8
Change-Id: I16c99c4d6482a0b94a9a39af360638ec438873e6
Reviewed-by: Alexey Edelev <alexey.edelev@qt.io>
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>

src/protobufqttypes/doc/protobufqtcoretypes.qdoc

@@ -75,4 +75,15 @@
     The Qt Protobuf generator detects Qt Core types, supplied by the
     ProtobufQtCoreTypes package, and uses them directly. This gives
     you the flexibility to use these types without additional conversion.
+
+    In CMake, you can extract the proto include path from the library using the
+    \c{QT_PROTO_INCLUDES} target property:
+    \badcode
+        qt_add_protobuf(MyProtoLib
+            PROTO_FILES
+                MyQtCoreTypesUsage.proto
+            PROTO_INCLUDES
+                $<TARGET_PROPERTY:Qt::ProtobufQtCoreTypes,QT_PROTO_INCLUDES>
+        )
+    \endcode
 */

src/protobufqttypes/doc/protobufqtguitypes.qdoc

@@ -69,6 +69,17 @@
     ProtobufQtGuiTypes package, and uses them directly. This gives
     you the flexibility to use these types without additional conversion.
 
+    In CMake, you can extract the proto include path from the library using the
+    \c{QT_PROTO_INCLUDES} target property:
+    \badcode
+        qt_add_protobuf(MyProtoLib
+            PROTO_FILES
+                MyQtGuiTypesUsage.proto
+            PROTO_INCLUDES
+                $<TARGET_PROPERTY:Qt::ProtobufQtGuiTypes,QT_PROTO_INCLUDES>
+        )
+    \endcode
+
     \section2 QImage serialization
     The ProtobufQtGuiTypes library provides the possibility to work with the
     QImage class. Usually, the library converts all QImage objects in