doc: explain ClientRequest#setTimeout time unit

State a time unit for the timeout parameter in ClientRequest#setTimeout

PR-URL: nodejs#4458
Reviewed-By: Stephen Belanger <admin@stephenbelanger.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/http.markdown

@@ -375,6 +375,9 @@ Once a socket is assigned to this request and is connected
 Once a socket is assigned to this request and is connected
 [`socket.setTimeout()`][] will be called.
 
+* `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.
+
 ### request.write(chunk[, encoding][, callback])
 
 Sends a chunk of the body.  By calling this method