Document `TraitType.read_only`

By searching the repo, I was able to find this description in the changelog:

> Trait attributes initialized with read_only=True can only be set with the set_trait method. Attempts to directly modify a read-only trait attribute raises a TraitError.

This description would be nice to have in docs/docstrings!

在仓库中搜索 TraitType.read_only 和 changelog 描述，然后检查相关的 docs 或 docstrings。记录只读 trait 属性只能通过 set_trait 设置，直接修改会引发 TraitError；当 TraitType 的文档中涵盖了这一行为时即完成。

由索引模型根据 Issue 内容生成。