notes: document change in nix-shell behavior

doc/manual/rl-next/shebang-relative.md

@@ -0,0 +1,8 @@
+synopsis: ensure nix-shell shebang uses relative path
+prs: #5088
+description: {
+
+`nix-shell` shebangs use the script file's relative location to resolve relative paths to files passed as command line arguments, but expression arguments were still evaluated using the current working directory as a base path.
+The new behavior is that evalutations are performed relative to the script.
+
+}