doc,test: minor improvements to O_DSYNC

PR-URL: https://github.com/nodejs/node/pull/15547
Refs: https://github.com/nodejs/node/pull/15451


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>