doc: clarify descriptions of _writev chunks argument

Fixes: https://github.com/nodejs/node/issues/36789


Signed-off-by: James M Snell <jasnell@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/36822


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Daijiro Wachi <daijiro.wachi@gmail.com>