doc: mention callback for http res/req write & end
doc: mention callback for http res/req write & end

Add documentation for the callback parameter of http.ClientRequest's and
http.ServerResponse's write and end methods.

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node