[PATCH] add documentation about backend naming for VMOD authors

[Dridi Boukelmoune]

>> * struct director without a struct backend do not appear in
>> CLI/VSM unless the VMOD implements all the stuff for this.
>
> What about a VMOD for backends that don't speak HTTP over TCP? I have
> assumed that these would be directors without struct backend (or is
> that assumption wrong?), but I don't see why they shouldn't be allowed
> to VRT_register() to get the "official" CLI and VSM support.

Your assumption is right, AFAICT.