doc: http: document ServerResponse 'finish' event

doc/api/http.markdown

@@ -235,6 +235,17 @@ The response implements the [Writable Stream][] interface. This is an
 Indicates that the underlying connection was terminated before
 [response.end()][] was called or able to flush.
 
+### Event: 'finish'
+
+`function () { }`
+
+Emitted when the response has been sent. More specifically, this event is
+emitted when the last segment of the response headers and body have been
+handed off to the operating system for transmission over the network. It
+does not imply that the client has received anything yet.
+
+After this event, no more events will be emitted on the response object.
+
 ### response.writeContinue()
 
 Sends a HTTP/1.1 100 Continue message to the client, indicating that