Add missing is_deprecated flag on the SurfaceTool.generate_lod function.

(cherry picked from commit 8153739df987dad5e8c6e4dff87416332b6ef47e)

doc/classes/SurfaceTool.xml

@@ -118,7 +118,7 @@
 				Removes the index array by expanding the vertex array.
 			</description>
 		</method>
-		<method name="generate_lod">
+		<method name="generate_lod" is_deprecated="true">
 			<return type="PackedInt32Array" />
 			<param index="0" name="nd_threshold" type="float" />
 			<param index="1" name="target_index_count" type="int" default="3" />