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: #19752
Refs: 80ea0c5
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/stream.md

@@ -1520,13 +1520,12 @@ buffered. When the `callback` is invoked, the stream might emit a [`'drain'`][]
 event. If a stream implementation is capable of processing multiple chunks of
 data at once, the `writable._writev()` method should be implemented.
 
-If the `decodeStrings` property is set in the constructor options, then
-`chunk` may be a string rather than a Buffer, and `encoding` will
-indicate the character encoding of the string. This is to support
-implementations that have an optimized handling for certain string
-data encodings. If the `decodeStrings` property is explicitly set to `false`,
-the `encoding` argument can be safely ignored, and `chunk` will remain the same
-object that is passed to `.write()`.
+If the `decodeStrings` property is explicitly set to `false` in the constructor
+options, then `chunk` will remain the same object that is passed to `.write()`,
+and may be a string rather than a `Buffer`. This is to support implementations
+that have an optimized handling for certain string data encodings. In that case,
+the `encoding` argument will indicate the character encoding of the string.
+Otherwise, the `encoding` argument can be safely ignored.
 
 The `writable._write()` method is prefixed with an underscore because it is
 internal to the class that defines it, and should never be called directly by