Document the difference between Fiber and Call/CC

[naman-dixit-srib]

The documentation and example code for both is near identical, and there is no explicitly documented difference between the two (or between them and execution_context and coroutine).

Could this be rectified, as it is very hard to those unfamiliar with this particular library to figure what subtle difference is there between all these alternatives?

[olk]

call/cc is based on an historic API fiber_context is the API proposed in "P0876: fiber_context - fibers without scheduler" (C++ standardisation).