docs(iroh-net): Explain when sockets are closed (#2892)

## Description

This explains when the UDP sockets are closed in the Endpoint::close
docs.


## Breaking Changes

<!-- Optional, if there are any breaking changes document them,
including how to migrate older code. -->

## Notes & open questions

<!-- Any notes, remarks or open questions you have to make about the PR.
-->

## Change checklist

- [x] Self-review.
- [x] Documentation updates following the [style
guide](https://rust-lang.github.io/rfcs/1574-more-api-documentation-conventions.html#appendix-a-full-conventions-text),
if relevant.
- [x] Tests if relevant.
- [x] All breaking changes documented.

Author: flub

iroh-net/src/endpoint.rs

@@ -957,8 +957,10 @@ impl Endpoint {
     /// This will close all open QUIC connections with the provided error_code and
     /// reason. See [`quinn::Connection`] for details on how these are interpreted.
     ///
-    /// It will then wait for all connections to actually be shutdown, and afterwards
-    /// close the magic socket.
+    /// It will then wait for all connections to actually be shutdown, and afterwards close
+    /// the magic socket.  Be aware however that the underlying UDP sockets are only closed
+    /// on [`Drop`], bearing in mind the [`Endpoint`] is only dropped once all the clones
+    /// are dropped.
     ///
     /// Returns an error if closing the magic socket failed.
     /// TODO: Document error cases.