docs: improved documentation for el-form (#12635)

* Docs: Document el-form label-position attribute

Adds improved documentation to the el-form `label-position` attribute, which behaves differently when used with and without `label-width`. This has been a source of confusion for myself and others:

https://github.com/ElemeFE/element/issues/8477

Thanks!

* Document el-form label-position attribute (zh-CN, needs translation)

* Document el-form label-position attribute (es)

* Update form.md