Document req.next

[crandmck]

Per discussion in TC meeting we should doc req.next with some disclaimer about using it.

[crandmck]

@dougwilson I recall that we discussed this, but not the details. What kind of disclaimer should we include with the doc for this function?

[dougwilson]

A disclaimer probably saying that req.next should only be used as a last resort, and that there are various edge cases where it may not actually refer to what a user would think of as next(). Passing around callbacks would be recommended over this.