Docs: MeshDataTool: showcase tool in code example

(cherry picked from commit 4f9b993423e803bfda965bb5dcd5dcc00f2f3395)

doc/classes/MeshDataTool.xml

@@ -8,14 +8,21 @@
 		To use MeshDataTool, load a mesh with [method create_from_surface]. When you are finished editing the data commit the data to a mesh with [method commit_to_surface].
 		Below is an example of how MeshDataTool may be used.
 		[codeblock]
+		var mesh = ArrayMesh.new()
+		mesh.add_surface_from_arrays(Mesh.PRIMITIVE_TRIANGLES, CubeMesh.new().get_mesh_arrays())
 		var mdt = MeshDataTool.new()
 		mdt.create_from_surface(mesh, 0)
 		for i in range(mdt.get_vertex_count()):
 		    var vertex = mdt.get_vertex(i)
-		    ...
+		    # In this example we extend the mesh by one unit, which results in seperated faces as it is flat shaded.
+		    vertex += mdt.get_vertex_normal(i)
+		    # Save your change.
 		    mdt.set_vertex(i, vertex)
 		mesh.surface_remove(0)
 		mdt.commit_to_surface(mesh)
+		var mi = MeshInstance.new()
+		mi.mesh = mesh
+		add_child(mi)
 		[/codeblock]
 		See also [ArrayMesh], [ImmediateGeometry] and [SurfaceTool] for procedural geometry generation.
 		[b]Note:[/b] Godot uses clockwise [url=https://learnopengl.com/Advanced-OpenGL/Face-culling]winding order[/url] for front faces of triangle primitive modes.