doc: clarify os.cpus() returns logical CPU cores

Original text implied it returns physical cpu cores. This is misleading
as `os.cpus().length` is often used to get the CPU core count in
Node.js. In reality that will return the thread count which may not be
what the user intended.

PR-URL: https://github.com/nodejs/node/pull/16282
Fixes: https://github.com/nodejs/node/issues/16279


Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>