doc for WeChatScene

lib/src/wechat_scene.dart

-enum WeChatScene { SESSION, TIMELINE, FAVORITE }
+///[WeChatScene.SESSION]会话
+///[WeChatScene.TIMELINE]朋友圈
+///[WeChatScene.FAVORITE]收藏
+enum WeChatScene {
+  SESSION,
+  TIMELINE,
+  FAVORITE }