Options for multiline completions

Hi
I would like to be able to have a way to display multiline data on the meta property of a completion, ideally it'd be a secondary window like in neovim:

but if that's too complicated it can also be useful in a bottom/top bar (classic vim):

I've hacked a bit some examples and came up with this:

however I don't see a way to make the metadata not show in the prompt completion, and having it visible twice is not great

This image comes from:

def bottom_toolbar():
    cs = get_app().current_buffer.complete_state
    if cs and cs.current_completion:
        cc = cs.current_completion
        return cc.display_meta
    else:
        return None


def main():
    sess = PromptSession(
        completer=animal_completer,
        bottom_toolbar=bottom_toolbar,
    )
    text = sess.prompt(
        "Give some animals: ",
    )
    print("You said: %s" % text)

I would also be OK if I can somehow generate a floating window and stick it next to the completions -- but I haven't figured out how to do this yet

Start by tracing how completion metadata is exposed through current_completion and display_meta, using the bottom_toolbar() example in the issue as the entry point. Compare the requested secondary-window and top/bottom-bar presentation options, then define an implementation approach that avoids showing the metadata twice. Done should include a documented, tested way to display multiline completion metadata.