doc: improve buf.fill() documentation

* Improve prepositions.
* Wrap at 80 characters.
* Formalize colloquial text.
* Remove unnecessary sentence.
* Use colons to introduce example code.

PR-URL: https://github.com/nodejs/node/pull/19846


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>