`fiber_cancel()` C API clarification

[TarantoolBot]

Root document: https://www.tarantool.io/en/doc/latest/reference/reference_capi/fiber/
Product: Tarantool
SME: @ Gerold103

The documentation must warn that the fiber passed to
fiber_cancel() must not be already dead unless it was set to be
joinable. Same for fiber_wakeup() and all the other fiber
functions. A dead non-joinable fiber could already be freed or
reused.
Requested by @Gerold103 in tarantool/tarantool@dbb9027.