[doc] clarify the use of dynamic-wind, probably in the Barriers chapter

[attila-lendvai]

i'm new to scheme, but i think it's not possible to use constructs like call-with-input-file because it has a dynamic-wind inside of it that closes the file/port when a fiber suspends.

if i'm right, then please mention this in the docs. it would have saved me some time.

[emixa-d]

‘bug’, because Guile should also use a different dynamic-wind (see: https://github.com/wingo/fibers/pull/73).