[libc++] Document implementation-defined behaviour of filebuf::sync()

https://eel.is/c++draft/filebuf#virtuals-19 states that a flebuf with a get area causes implementation-defined behaviour. We should document that behaviour.

Start by locating libc++'s filebuf::sync() implementation and compare its get-area behavior with the C++ draft section at eel.is/c++draft/filebuf#virtuals-19. Identify the implementation-defined behavior that should be documented, then update the relevant libc++ documentation and verify that it accurately describes the implementation.

Written by the indexing model from the issue text.