Add information about frozen stdlib modules.

With the more stdlib modules getting frozen recently, it would be useful to have a section (or page) somewhere about:

• what are frozen modules?
• the different kinds of frozen modules (bootstrap vs. stdlib vs. testing)
• how to identify or modify which stdlib modules are frozen
• why some modules are frozen and some aren't
• how to deal with frozen modules in tests
• any other helpful info

未指定文件、测试或文档入口。首先查看 devguide 现有的文档结构以及有关冻结模块的材料，然后确定此主题适合放在哪里。完成的标准是有一个页面或章节说明所列出的冻结模块类型、如何识别和修改它们、冻结的理由，以及测试指南。