libtool2: add bootstrapping warning

pkgs/development/tools/misc/libtool/libtool2.nix

@@ -1,6 +1,11 @@
 { lib, stdenv, fetchurl, fetchpatch, autoconf, automake, m4, perl, help2man
 }:
 
+# Note: this package is used for bootstrapping fetchurl, and thus
+# cannot use fetchpatch! All mutable patches (generated by GitHub or
+# cgit) that are needed here should be included directly in Nixpkgs as
+# files.
+
 stdenv.mkDerivation rec {
   pname = "libtool";
   version = "2.4.6";