doc: add note about mkdtemp() platform differences

PR-URL: https://github.com/nodejs/node/pull/26944
Fixes: https://github.com/nodejs/node/issues/26435


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Signed-off-by: Beth Griggs <Bethany.Griggs@uk.ibm.com>