lib: fix isReadable and isWritable return type value

PR-URL: #59089
Fixes: #59006
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Edy Silva <edigleyssonsilva@gmail.com>
Reviewed-By: Ethan Arrowood <ethan@arrowood.dev>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Mattias Buelens <mattias@buelens.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: joaoGabriel55

doc/api/stream.md

@@ -3074,10 +3074,17 @@ changes:
 -->
 
 * `stream` {Readable|Duplex|ReadableStream}
-* Returns: {boolean}
+* Returns: {boolean|null} - Only returns `null` if `stream` is not a valid `Readable`, `Duplex` or `ReadableStream`.
 
 Returns whether the stream is readable.
 
+### `stream.isWritable(stream)`
+
+* `stream` {Writable|Duplex|WritableStream}
+* Returns: {boolean|null} - Only returns `null` if `stream` is not a valid `Writable`, `Duplex` or `WritableStream`.
+
+Returns whether the stream is writable.
+
 ### `stream.Readable.from(iterable[, options])`
 
 <!-- YAML