doc: improve buf.indexOf() documentation style

* improve usage of "coerce" in buffer.md
* reduce informal tone in buffer.md
* wrap line at 80 characters in buffer.md
* wrap value in backtick for consistency

PR-URL: https://github.com/nodejs/node/pull/19861


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>