Add docs for #[rpc]

[Bromeon]

Adds #[godot_api] level documentation for the #[rpc] attribute added in #807.
Moves RpcConfig from godot::meta to godot::register.

Also mentions that you can use functions instead of constants, which allow the use of the Default trait:

fn provide_cfg() -> RpcConfig {
    RpcConfig {
        transfer_mode: TransferMode::RELIABLE,
        channel: 1,
        ..Default::default()
    }
}

#[godot_api]
impl RpcTest {
    #[rpc(config = provide_cfg())]
    pub fn my_rpc_method(&mut self) {}
}

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-949