doc: clarify FileHandle text

There is a paragraph about why the promise-based API uses `FileHandle`
rather than a numeric `fd`. Unfortunately, the paragraph is a bit of
word-salad. Edit it for clarity, grammar, and style.

PR-URL: https://github.com/nodejs/node/pull/20450


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>