lib: fix isReadable and isWritable return type value

lib: fix isReadable and isWritable return type value · nodejs/node@4fd213c

Original file line number	Diff line number	Diff line change
`@@ -3074,10 +3074,17 @@ changes:`
`3074`	`3074`	`-->`
`3075`	`3075`
`3076`	`3076`	* `stream` {Readable\|Duplex\|ReadableStream}
`3077`		`-* Returns: {boolean}`
	`3077`	+* Returns: {boolean\|null} - Only returns `null` if `stream` is not a valid `Readable`, `Duplex` or `ReadableStream`.
`3078`	`3078`
`3079`	`3079`	`Returns whether the stream is readable.`
`3080`	`3080`
	`3081`	+### `stream.isWritable(stream)`
	`3082`	`+`
	`3083`	+* `stream` {Writable\|Duplex\|WritableStream}
	`3084`	+* Returns: {boolean\|null} - Only returns `null` if `stream` is not a valid `Writable`, `Duplex` or `WritableStream`.
	`3085`	`+`
	`3086`	`+Returns whether the stream is writable.`
	`3087`	`+`
`3081`	`3088`	### `stream.Readable.from(iterable[, options])`
`3082`	`3089`
`3083`	`3090`	`<!-- YAML`