doc/rl-2.20: clarify builders-use-substitutes vs. substitute-on-desti…

…nation

...as this lead to confusion before.

(cherry picked from commit 50557ad)

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

@@ -194,4 +194,5 @@
 - `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`.
+  Previously this behavior was controlled by the
+  `builders-use-substitutes` setting and `--substitute-on-destination` was ignored.