gprbuild: add comment on headerpad_max_install_names

pkgs/development/ada-modules/gprbuild/default.nix

@@ -42,6 +42,8 @@ stdenv.mkDerivation {
   ];
 
   env = lib.optionalAttrs stdenv.isDarwin {
+    # Ensure that there is enough space for the `fixDarwinDylibNames` hook to
+    # update the install names of the output dylibs.
     NIX_LDFLAGS = "-headerpad_max_install_names";
   };