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