lib: fix isReadable and isWritable return type value

PR-URL: nodejs#59089
Fixes: nodejs#59006
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Edy Silva <[email protected]>
Reviewed-By: Ethan Arrowood <[email protected]>
Reviewed-By: James M Snell <[email protected]>
Reviewed-By: Mattias Buelens <[email protected]>
Reviewed-By: Luigi Pinca <[email protected]>

Author: joaoGabriel55

doc/api/stream.md

@@ -3084,10 +3084,17 @@ 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