`#[juniper::object]` interfaces attribute not documented

[jacobh]

Hi,

this afternoon I've been porting some graphql types to using the attribute macro syntax. I was stuck trying to get my interfaces working correctly.

I think I've figured it out, with interfaces now being an attribute of the macro, but I couldn't see any mention of that syntax anywhere in the docs, and had to dig into the source.

Other than this I'm very much liking the attribute macro syntax! reads much more nicely, and rustfmt works correctly.

Old:

graphql_object!(DynamicQueueContext: Context as "DynamicQueue" |&self| {
    interfaces: [Box<dyn QueueContext>]

    // Queue interface fields
    field id() -> Uuid {
        ContextItem::id(self)
    }
    field name() -> String {
        self.name()
    }
    ...
}

New:

#[juniper::object(
    name = "DynamicQueue"
    Context = Context
    interfaces = [Box<dyn QueueContext>]
)]
impl DynamicQueueContext {
    // Queue interface fields
    fn id(&self) -> Uuid {
        ContextItem::id(self)
    }
    fn name(&self) -> String {
        self.name()
    }
    ...
}

[tyranron]

Now documented in Book along with interfaces.