[PATCH] add documentation about backend naming for VMOD authors

[Dridi Boukelmoune]

> Comments ?
>
> (After this is settled, we'll look at which parts we can also
> put in 4.1.x in ABI/API compatible ways)

8. set the default backend/director in vcl_init

set default_backend = rr.backend();

The default backend cannot be removed. If no default backend is set,
the first one is picked unless there's one actually named "default".

9. remove the need for at least one native backend

As long as a at least one backend is registered when the VCL is
loaded, a user should not need to declare a dummy backend when they
only use VMOD backends.

Best,
Dridi