doc: be concise about serial commas

There is no need to hedge bets on the serial comma. Simply instruct
people to use it.

PR-URL: https://github.com/nodejs/node/pull/17384


Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Alexey Orlenko <eaglexrlnk@gmail.com>
Reviewed-By: Refael Ackermann <refack@gmail.com>
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Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jon Moss <me@jonathanmoss.me>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
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Reviewed-By: Tobias Nießen <tniessen@tnie.de>