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

PR-URL: https://github.com/nodejs/node/pull/28224
Refs: https://github.com/nodejs/node/commit/ae6c7044c8d48cc8e7c267efca7429d5aa4df993


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>