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

未指定檔案、測試或文件入口。首先檢視 devguide 現有的文件結構以及關於凍結模組的資料，然後決定此主題適合放在哪裡。完成的標準是有一個頁面或區段說明所列出的凍結模組類型、如何識別和修改它們、凍結的理由，以及測試指南。