This covers both generic callbacks (adapt_callbacks.cpp,
adapt_method_calls.cpp) and custom Asio completion tokens (yield.hpp,
promise_completion_token.hpp, detail/yield.hpp, detail/promise_handler.hpp).
Mark up the relevant source files to provide code snippets for callbacks.qbk.
This illustrates how to interface a synchronous Fiber function with an async
API whose notification consists of an abstract base class with virtual
success/error methods.
