doc: explain _writev() API

The exact context of invocation of _writev API is not well known.
Also, the choice between _write and _writev is not well known.
Add a description to make it explicit.

Fixes: https://github.com/nodejs/node/issues/28408
Refs: https://github.com/nodejs/node/pull/28690



Co-authored-by: Parker Bjur <bjur.parker45@gmail.com>

PR-URL: https://github.com/nodejs/node/pull/31356


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>