model中 constants 和 privates 的正确使用方式

从源码中 [`create-modal.js`](https://github.com/alibaba/vanex/blob/ef541f6fb1ee1c510bab99204432a4b7aa46ac8b/src/create-model.js#L23-L24) 看到 `constants` , `privates`，文档里没有说明他们主要的用法或者主要应用场景。能说明下吗？我要完全拥抱 vanex

Start with src/create-model.js at the constants and privates references linked in the issue, then review the existing documentation structure for the appropriate place to add guidance. The work is done when the main usage patterns and application scenarios are documented with enough context for a Vanex user to apply them.

Written by the indexing model from the issue text.