changed description of `merge_polygons`

Clarified that the `merge_polygons` method can produce multiple holes.

doc/classes/Geometry2D.xml

@@ -184,7 +184,7 @@
 			</argument>
 			<description>
 				Merges (combines) [code]polygon_a[/code] and [code]polygon_b[/code] and returns an array of merged polygons. This performs [constant OPERATION_UNION] between polygons.
-				The operation may result in an outer polygon (boundary) and inner polygon (hole) produced which could be distinguished by calling [method is_polygon_clockwise].
+				The operation may result in an outer polygon (boundary) and multiple inner polygons (holes) produced which could be distinguished by calling [method is_polygon_clockwise].
 			</description>
 		</method>
 		<method name="offset_polygon">