Link specifically to req/res in unclear node http module documentation

[morkeltry]

In week 7, we found that it is difficult for people to find the authoritative documentation on request objects, as they are described in the node docs as message or IncomingMessage (which is a type which could be either ClientRequest or ServerResponse )
Proposal:
In 'What is a handler function?' in Step 3, include:

(request and response are instances of the two different types of IncomingMessage object)