Fix documentation for Fiber#transfer [ci skip]

Fiber#transfer prevents calling Fiber#resume on the receiver of the
transfer method, not the fiber calling transfer.

Transfering back to a fiber does not allow later calling resume on
the fiber.  Once transfer has been called on a fiber, you can never
call resume on the fiber.

Calling resume on a transferred fiber is not a double resume error,
it is a different FiberError (cannot resume transferred Fiber).

For details on the differences between transferred fibers and
regular fibers, see Sasada-san's RubyKaigi 2017 presentation (in
short, Fiber#transfer is for coroutine, Fiber#resume is for
semi-coroutine).

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