Add inline help for field attributes

When adding custom fields to a post (e.g. {_field}), some attributes are supported. These should be added to the inline help next to the editor (where the names of the custom fields are shown as well).

Here an idea how it could be implemented: https://github.com/wp-plugins/my-content-management/pull/1

https://ww.wp.xz.cn/plugins/my-content-management/