doc: update npm-start.md

Fixes #9460, specifying the default command run by "npm start" and the
fact that you can pass it arguments.

PR-URL: #10097

doc/cli/npm-start.md

@@ -7,7 +7,13 @@ npm-start(1) -- Start a package
 
 ## DESCRIPTION
 
-This runs a package's "start" script, if one was provided.
+This runs an arbitrary command specified in the package's `"start"` property of
+its `"scripts"` object. If no `"start"` property is specified on the
+`"scripts"` object, it will run `node server.js`.
+
+As of [`npm@2.0.0`](http://blog.npmjs.org/post/98131109725/npm-2-0-0), you can
+use custom arguments when executing scripts. Refer to npm-run-script(1) for
+more details.
 
 ## SEE ALSO