doc: improve buf.lastIndexOf() text

Improve description of `encoding` option. (This change is also applied
to `buf.indexOf()`.) Improve punctuation. Wrap at 80 characters.

PR-URL: https://github.com/nodejs/node/pull/19904


Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>