Document `SurfaceTool.generate_normals()` interaction with smooth groups

doc/classes/SurfaceTool.xml

@@ -133,6 +133,7 @@
 			<description>
 				Generates normals from vertices so you do not have to do it manually. If [param flip] is [code]true[/code], the resulting normals will be inverted. [method generate_normals] should be called [i]after[/i] generating geometry and [i]before[/i] committing the mesh using [method commit] or [method commit_to_arrays]. For correct display of normal-mapped surfaces, you will also have to generate tangents using [method generate_tangents].
 				[b]Note:[/b] [method generate_normals] only works if the primitive type to be set to [constant Mesh.PRIMITIVE_TRIANGLES].
+				[b]Note:[/b] [method generate_normals] takes smooth groups into account. If you don't specify any smooth group for each vertex, [method generate_normals] will smooth normals for you.
 			</description>
 		</method>
 		<method name="generate_tangents">