expand-response-params: explain what a "response file" is

Co-authored-by: Adam Joseph <adam@westernsemico.com>

pkgs/build-support/expand-response-params/default.nix

@@ -1,5 +1,12 @@
 { stdenv }:
 
+# A "response file" is a sequence of arguments that is passed via a
+# file, rather than via argv[].
+
+# For more information see:
+# https://gcc.gnu.org/wiki/Response_Files
+# https://www.intel.com/content/www/us/en/docs/dpcpp-cpp-compiler/developer-guide-reference/2023-0/use-response-files.html
+
 stdenv.mkDerivation {
   name = "expand-response-params";
   src = ./expand-response-params.c;