doc: remove "note that" from fs doc

Remove "note that" from the fs documentation, along with other minor
nearby improvements.

Before:

    Note that some characters are obscured by Strong Bad's head.

After:

    Some characters are obscured by Strong Bad's head.

PR-URL: https://github.com/nodejs/node/pull/21646


Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Сковорода Никита Андреевич <chalkerx@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>