BufferGeometry.setIndex: Improve type definition and docs

src/core/BufferGeometry.d.ts

@@ -48,8 +48,8 @@ export class BufferGeometry extends EventDispatcher {
 	userData: {[key: string]: any};
 	isBufferGeometry: boolean;
 
-	getIndex(): BufferAttribute;
-	setIndex( index: BufferAttribute | number[] ): void;
+	getIndex(): BufferAttribute | null;
+	setIndex( index: BufferAttribute | number[] | null ): void;
 
 	setAttribute( name: string, attribute: BufferAttribute | InterleavedBufferAttribute ): BufferGeometry;
 	getAttribute( name: string ): BufferAttribute | InterleavedBufferAttribute;