doc: Document http.request protocol option

Reviewed-By: James M Snell <jasnell@gmail.com>
PR-URL: nodejs/node-v0.x-archive#25591

Author: scop

doc/api/http.markdown

@@ -457,6 +457,7 @@ automatically parsed with [url.parse()][].
 
 Options:
 
+- `protocol`: Protocol to use. Defaults to `'http:'`.
 - `host`: A domain name or IP address of the server to issue the request to.
   Defaults to `'localhost'`.
 - `hostname`: To support `url.parse()` `hostname` is preferred over `host`