Add information about frozen stdlib modules.

[ericsnowcurrently]

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