bashInteractive: add a warning not to be renamed/removed

pkgs/top-level/all-packages.nix

···
     interactive = stdenv.isCygwin; # patch for cygwin requires readline support
   });
 
+  # WARNING: this attribute is used by nix-shell so it shouldn't be removed/renamed
   bashInteractive = appendToName "interactive" (callPackage ../shells/bash/4.4.nix {
     interactive = true;
   });