make-options-doc: Make variablelist id configurable

I've tried XInclude set-xml-id first, but our tooling did not pick up on it.

nixos/lib/make-options-doc/default.nix

@@ -22,6 +22,10 @@
 , transformOptions ? lib.id  # function for additional tranformations of the options
 , documentType ? "appendix" # TODO deprecate "appendix" in favor of "none"
                             #      and/or rename function to moduleOptionDoc for clean slate
+
+  # If you include more than one option list into a document, you need to
+  # provide different ids.
+, variablelistId ? "configuration-variable-list"
 , revision ? "" # Specify revision for the options
 # a set of options the docs we are generating will be merged into, as if by recursiveUpdate.
 # used to split the options doc build into a static part (nixos/modules) and a dynamic part
@@ -177,6 +181,7 @@ in rec {
     ${pkgs.libxslt.bin}/bin/xsltproc \
       --stringparam documentType '${documentType}' \
       --stringparam revision '${revision}' \
+      --stringparam variablelistId '${variablelistId}' \
       -o intermediate.xml ${./options-to-docbook.xsl} sorted.xml
     ${pkgs.libxslt.bin}/bin/xsltproc \
       -o "$out" ${./postprocess-option-descriptions.xsl} intermediate.xml