docs: remote_node_id fails on the accept side for a 0-rtt connection

[rklaehn]

If you use Connecting::into_0rtt on a Connecting that comes from Endpoint::accept and get an Ok(Connection), calling remote_node_id immediately afterwards will fail.

This is independent of if the other side has actually tried or succeeded to connect via 0-rtt. I get this even with a fresh client that obviously can't use 0-rtt yet, and got it even before I did the .into_0rtt dance on the client side at all.

I manually tried, and it turns out that the first location where you can call remote_node_id is after calling Connection::accept_bi.

We need to document this behaviour so people don't get confused, and also need to keep remote_node_id returning a Result.

[b5]

related to worke happening in #3123