#### What this PR does / why we need it:
为 formkit select 组件的远程搜索,增加 `fieldSelectorKey` 字段的使用说明。
see https://github.com/halo-dev/halo/pull/6591
#### Does this PR introduce a user-facing change?
```release-note
None
```
#### What this PR does / why we need it:
补充 formkit select 相关文档。
#### Which issue(s) this PR fixes:
Fixes#408
#### Does this PR introduce a user-facing change?
```release-note
None
```