Document need for req.user in examples

[robertjd]

Some of our examples show references to req.user, but it's not clear that you have to use the getUser middleware in order for this value to be available. Here is one example:

https://docs.stormpath.com/nodejs/express/latest/authentication.html#cookie-authentication

We should find other places where this is an issue and fix the documentation to make this more clear.

[mdeggies]

https://github.com/stormpath/express-stormpath/pull/547