Define return type of clientRequest.setTimeout

nodejs#1699 did not update documentation

Author: MikeRalphson

doc/api/http.md

@@ -496,6 +496,8 @@ Once a socket is assigned to this request and is connected
 * `timeout` {Number} Milliseconds before a request is considered to be timed out.
 * `callback` {Function} Optional function to be called when a timeout occurs. Same as binding to the `timeout` event.
 
+Returns `request`.
+
 ### request.write(chunk[, encoding][, callback])
 <!-- YAML
 added: v0.1.29