doc: clarify transform._transform() callback argument logic

Clarify that `transform._transform()` callback second argument is
used only if the first argument is `null`, i.e. no error occured
processing the chunk.

PR-URL: nodejs#48680
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Akhil Marsonya <akhil.marsonya27@gmail.com>

doc/api/stream.md

@@ -4517,7 +4517,8 @@ The `callback` function must be called only when the current chunk is completely
 consumed. The first argument passed to the `callback` must be an `Error` object
 if an error occurred while processing the input or `null` otherwise. If a second
 argument is passed to the `callback`, it will be forwarded on to the
-`transform.push()` method. In other words, the following are equivalent:
+`transform.push()` method, but only if the first argument is falsy. In other
+words, the following are equivalent:
 
 ```js
 transform.prototype._transform = function(data, encoding, callback) {