doc: edit subpath export patterns introduction

* Use parallel construction in the two sentences
* Backticks around _package.json_ to match rest of file
* Add comma for readability
* Own the recommendation ("we recommend")

PR-URL: https://github.com/nodejs/node/pull/35254


Reviewed-By: Jan Krems <jan.krems@gmail.com>
Reviewed-By: Michael Dawson <midawson@redhat.com>
Reviewed-By: Guy Bedford <guybedford@gmail.com>