doc: add note about fs.close undefined behavior

Basically what the close(2) man page says – if it’s possible that the fd is being used for a syscall in one thread of the threadpool, we should not be calling close() concurrently.

Refs: https://github.com/nodejs/node/issues/30864

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines