docs(affix): fix listen-to

demo/documentation/components/affix/enUS/index.demo-entry.md

@@ -9,7 +9,7 @@ position
 |Name|Type|Default|Description|
 |-|-|-|-|
 |bottom|`number`|`undefined`|The css bottom property after trigger bottom affix. (if not set, use `offset-bottom` prop)|
-|listen-to|`() => HTMLElement`|a function that returns the nearest scrollable ascendant element|The scrolling element to listen scrolling.|
+|listen-to|`string \| HTMLElement`|`undefined`|The scrolling element to listen scrolling. If not set it will listen to the nearest scrollable ascendant element.|
 |offset-bottom|`number`|`undefined`|The distance px to bottom of target to trigger bottom affix. (if not set, use `bottom` prop)|
 |offset-top|`number`|`undefined`|The distance px to top of target to trigger top affix. (if not set, use `top` prop)|
 |position|`'fixed' \| 'absolute'`|`'fixed'`||

demo/documentation/components/affix/zhCN/index.demo-entry.md

@@ -9,7 +9,7 @@ position
 |名称|类型|默认值|描述|
 |-|-|-|-|
 |bottom|`number`|`undefined`|在触发顶部固定后 Affix 的 CSS bottom 属性（如果没设定，会使用 `offset-bottom` 代替)|
-|listen-to|`() => HTMLElement`|一个返回最邻近可滚动祖先元素的函数|需要监听滚动的元素|
+|listen-to|`string \| HTMLElement`|`undefined`|需要监听滚动的元素，如果未设定则会监听最近的可滚动祖先元素|
 |offset-bottom|`number`|`undefined`|触发底部固定时，Affix 和目标元素元素的底部距离（如果没设定，会使用 `bottom` 代替)|
 |offset-top|`number`|`undefined`|触发顶部固定时，Affix 和目标元素元素的顶部距离（如果没设定，会使用 `top` 代替)|
 |position|`'fixed' \| 'absolute'`|`'fixed'`||

src/affix/src/Affix.vue

@@ -26,7 +26,7 @@ export default {
   props: {
     listenTo: {
       type: [String, Object],
-      default: null
+      default: undefined
     },
     offsetTop: {
       type: Number,