Streaming / Delayed docs should be updated to clarify where $headers is intended to come from

[markstos]

The Streaming section includes this code example:

my $app = sub {
    my $env = shift;

    # Delays response until it fetches content from the network
    return sub {
        my $responder = shift;

        fetch_content_from_server(sub {
            my $content = shift;
            $responder->([ 200, $headers, [ $content ] ]);
        });
    };
};

The docs should be updated to clarify which of the 3 levels of anonymous subs that $headers is expected to originate in.