doc/rl-2.20: add missing entry about nix copy --to ssh-ng://...

This requires `--substitute-on-destination` if you want the remote side
to substitute instead of copying if possible.

For completeness sake, document it here.

Also, the stable Nix from nixpkgs is still 2.18, so more folks may
stumble upon this when this is bumped, so I'd expect this to be actually
useful.

Closes #10182

(cherry picked from commit f34b8de5b2)

doc/manual/src/release-notes/rl-2.20.md

@@ -190,3 +190,8 @@
   repository where the `flake.lock` file is `.gitignore`d
   [#8854](https://github.com/NixOS/nix/issues/8854)
   [#9324](https://github.com/NixOS/nix/pull/9324)
+
+- `nix copy` to a `ssh-ng` store now needs `--substitute-on-destination` (a.k.a. `-s`)
+  in order to substitute paths on the remote store instead of copying them.
+  The behavior is consistent with `nix copy` to a different kind of remote store.
+  Previously this behavior was controlled by `--builders-use-substitutes`.