fs: document the Date conversion in Stats objects

Document why the dates are calculated with the timestamp in Numbers + 0.5.

The comment was previously lost in a revert.

Refs: https://github.com/nodejs/node/commit/ae6c7044c8d48cc8e7c267efca7429d5aa4df993

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