doc: add return type of clientRequest.setTimeout

Refs: https://github.com/nodejs/node/pull/1699
PR-URL: https://github.com/nodejs/node/pull/8356
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>

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
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