docs: fix description of _writev()

The documentation gave the impression that when the _writev method is used, it is used instead of _write. However, when _writev is used, _write loads the first chunk of data, then _writev loads all the remaining bufferred chunks in the write queue. The docs have been changed to reflect this behavior.

Fixes: https://github.com/nodejs/node/issues/28408

Checklist

• documentation is changed or added
• commit message follows commit guidelines