doc: fix about `decodeStrings` property of `stream.Writable`

This fixes negation ommitted in a former commit.
It also simplifies the text in general.

PR-URL: https://github.com/nodejs/node/pull/19752
Refs: https://github.com/nodejs/node/commit/80ea0c5a64fe673011e8b1025de23a0ea7f3f7f8


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>