doc: remove unclear text from fs.write()

There is a paragraph explaining that `fs.write()` cannot write a
substring of the provided string. It's not clear what `buffer` refers
to in that paragraph or even what the purpose of the paragraph is.
There's no suggestion elsewhere that `fs.write()` should be expected to
have a substring feature. Remove the paragraph.

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>