bug#21894: escape continuation doc wrong about reinvokability

[Zefram]

The manual says

#                  Escape continuations are delimited continuations whose
# only use is to make a non-local exit--i.e., to escape from the current
# continuation.  Such continuations are invoked only once, and for this
# reason they are sometimes called "one-shot continuations".

O RLY?

scheme@(guile-user)> (use-modules (ice-9 control))
scheme@(guile-user)> (define cc #f)
scheme@(guile-user)> (list 'a (let/ec e (list 'b (e 
(call-with-current-continuation (lambda (c) (set! cc c) 0))))))
$1 = (a 0)
scheme@(guile-user)> (cc 1)
$2 = (a 1)
scheme@(guile-user)> (cc 2)
$3 = (a 2)

Clearly I have invoked this escape continuation, successfully, more
than once.  The semantics here are perfectly sensible, it's just
the documentation that's off the mark, because it ignores how escape
continuations interact with other kinds of continuation.  I suggest
changing "Such continuations are invoked only once" sentence to something
like

    Such continuations can only be invoked from within the dynamic
    extent of the call to which they will jump.  Because the jump
    ends that extent, if escape continuations are the only kind of
    continuations being used it is only possible to invoke an escape
    continuation at most once.  For this reason they are sometimes
    called "one-shot continuations", but that is a misnomer when other
    kinds of continuations are also in use.  Most kinds can reinstate a
    dynamic extent that has been exited, and if the extent of an escape
    continuation is reinstated then it can be invoked again to exit that
    extent again.  Conversely, an escape continuation cannot be invoked
    from a separate thread that has its own dynamic state not including
    the continuation's extent, even if the continuation's extent is
    still in progress in its original thread and the continuation has
    never been invoked.

-zefram