message::base() needs better docs

[vinniefalco]

There should be an example of use:

request<string_body> req;
...
std::cout << req.base(); // print just the header portion

Also we should group the overloads with defgroup or something so they don't require the overload page.

[jaredwy]

As per our conversation on slack. It would also be great to show an example of req[http::fields::*] as well.

[vinniefalco]

That could go in the description for message but keep in mind, that syntax only works when you are using basic_fields (the default Fields type).