doc: document the finish event

Since the stream implementer is not expected to overwrite
.end() the finish event is necessary in order to know when
no more data can be written

doc/api/stream.markdown

@@ -353,6 +353,11 @@ without buffering again. Listen for it when `stream.write()` returns
 Emitted when the underlying resource (for example, the backing file
 descriptor) has been closed. Not all streams will emit this.
 
+### Event: 'finish'
+
+When `end()` is called and there are no more chunks to write, this
+event is emitted.
+
 ### Event: 'pipe'
 
 * `source` {Readable Stream}