doc: shebang release notes, docs, tests

fix: release notes

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

@@ -1,5 +1,49 @@
 # Release X.Y (202?-??-??)
 
+- The experimental nix command is now a `#!-interpreter` by appending the
+  contents of any `#! nix` lines and the script's location to a single call.
+  Some examples:
+  ```
+  #!/usr/bin/env nix
+  #! nix shell --file "<nixpkgs>" hello --command bash
+
+  hello | cowsay
+  ```
+  or with flakes:
+  ```
+  #!/usr/bin/env nix
+  #! nix shell nixpkgs#bash nixpkgs#hello nixpkgs#cowsay --command bash
+
+  hello | cowsay
+  ```
+  or
+  ```bash
+  #! /usr/bin/env nix
+  #! nix shell --impure --expr
+  #! nix "with (import (builtins.getFlake ''nixpkgs'') {}); terraform.withPlugins (plugins: [ plugins.openstack ])"
+  #! nix --command bash
+
+  terraform "$@"
+  ```
+  or
+  ```
+  #!/usr/bin/env nix
+  //! ```cargo
+  //! [dependencies]
+  //! time = "0.1.25"
+  //! ```
+  /*
+  #!nix shell nixpkgs#rustc nixpkgs#rust-script nixpkgs#cargo --command rust-script
+  */
+  fn main() {
+      for argument in std::env::args().skip(1) {
+          println!("{}", argument);
+      };
+      println!("{}", std::env::var("HOME").expect(""));
+      println!("{}", time::now().rfc822z());
+  }
+  // vim: ft=rust
+  ```
 - [URL flake references](@docroot@/command-ref/new-cli/nix3-flake.md#flake-references) now support [percent-encoded](https://datatracker.ietf.org/doc/html/rfc3986#section-2.1) characters.
 
 - [Path-like flake references](@docroot@/command-ref/new-cli/nix3-flake.md#path-like-syntax) now accept arbitrary unicode characters (except `#` and `?`).