docs: add two props of treeselect (#1210)

- maxTagCount
- maxTagPlaceholder

components/tree-select/index.en-US.md

@@ -15,6 +15,8 @@
 | getPopupContainer | To set the container of the dropdown menu. The default is to create a `div` element in `body`, you can reset it to the scrolling area and make a relative reposition. | Function(triggerNode) | () => document.body |
 | labelInValue | whether to embed label in value, turn the format of value from `string` to `{value: string, label: VNode, halfChecked: string[]}` | boolean | false |
 | loadData | Load data asynchronously. | function(node) | - |
+| maxTagCount | Max tag count to show | number | - |
+| maxTagPlaceholder | Placeholder for not showing tags | slot/function(omittedValues) | - |
 | multiple | Support multiple or not, will be `true` when enable `treeCheckable`. | boolean | false |
 | placeholder | Placeholder of the select input | string\|slot | - |
 | searchPlaceholder | Placeholder of the search input | string\|slot | - |

components/tree-select/index.zh-CN.md

@@ -15,6 +15,8 @@
 | getPopupContainer | 菜单渲染父节点。默认渲染到 body 上，如果你遇到菜单滚动定位问题，试试修改为滚动的区域，并相对其定位。 | Function(triggerNode) | () => document.body |
 | labelInValue | 是否把每个选项的 label 包装到 value 中，会把 value 类型从 `string` 变为 `{value: string, label: VNode, halfChecked(treeCheckStrictly 时有效): string[] }` 的格式 | boolean | false |
 | loadData | 异步加载数据 | function(node) | - |
+| maxTagCount | 最多显示多少个 tag | number | - |
+| maxTagPlaceholder | 隐藏 tag 时显示的内容 | slot/function(omittedValues) | - |
 | multiple | 支持多选（当设置 treeCheckable 时自动变为true） | boolean | false |
 | placeholder | 选择框默认文字 | string\|slot | - |
 | searchPlaceholder | 搜索框默认文字 | string\|slot | - |