doc: add details for http res/req end callback

Add documentation for the callback parameter of http.ClientRequest's and
http.ServerResponse's end methods.

Signed-off-by: Julien Gilli <julien.gilli@joyent.com>

doc/api/http.markdown

@@ -444,6 +444,8 @@ response.
 If `data` is specified, it is equivalent to calling
 `response.write(data, encoding)` followed by `response.end(callback)`.
 
+If `callback` is specified, it will be called when the response stream
+is finished.
 
 ## http.request(options[, callback])
 
@@ -890,6 +892,9 @@ chunked, this will send the terminating `'0\r\n\r\n'`.
 If `data` is specified, it is equivalent to calling
 `request.write(data, encoding)` followed by `request.end(callback)`.
 
+If `callback` is specified, it will be called when the request stream
+is finished.
+
 ### request.abort()
 
 Aborts a request.  (New since v0.3.8.)