Geometry Nodes: show indicator when realize instances node is in legacy mode

For the majority of node groups created in Blender 3.0 the behavior does not change.
So far we only found a single file where this setting has an effect.

Differential Revision: https://developer.blender.org/D14078

source/blender/nodes/geometry/nodes/node_geo_realize_instances.cc

@@ -38,6 +38,15 @@ static void node_geo_exec(GeoNodeExecParams params)
 {
   const bool legacy_behavior = params.node().custom1 & GEO_NODE_REALIZE_INSTANCES_LEGACY_BEHAVIOR;
 
+  if (legacy_behavior) {
+    params.error_message_add(
+        NodeWarningType::Info,
+        TIP_("This node uses legacy behavior with regards to attributes on "
+             "instances. The behavior can be changed in the node properties in "
+             "the side bar. In most cases the new behavior is the same for files created in "
+             "Blender 3.0"));
+  }
+
   GeometrySet geometry_set = params.extract_input<GeometrySet>("Geometry");
   geometry::RealizeInstancesOptions options;
   options.keep_original_ids = legacy_behavior;