doc: document arguments for 'error' event on a stream

Fixes #6361.

doc/api/stream.markdown

@@ -183,6 +183,8 @@ descriptor) has been closed. Not all streams will emit this.
 
 #### Event: 'error'
 
+* {Error Object}
+
 Emitted if there was an error receiving data.
 
 #### readable.read([size])
@@ -579,6 +581,8 @@ reader.unpipe(writer);
 
 #### Event: 'error'
 
+* {Error object}
+
 Emitted if there was an error when writing or piping data.
 
 ### Class: stream.Duplex