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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines