doc: fs.mkdir('/') throws EPERM on Windows

Fixes: https://github.com/nodejs/node/issues/25110

PR-URL: https://github.com/nodejs/node/pull/25340


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>