nixos/manual: apply options preprocessing to full manual

the conversion to the markdown-based workflow missed that generating the
manual as docbook also generates the option docs with nixos-specific
wrapper elements that require postprocessing before the document is
real, valid docbook. restore this processing to the full manual.

it's not the prettiest thing, done like this, but we only need it for
one release so it doesn't have to be.

nixos/doc/manual/default.nix

@@ -166,7 +166,11 @@ let
         --manpage-urls ${manpageUrls} \
         --revision ${lib.escapeShellArg revision} \
         ./manual.md \
-        ./manual-combined.xml
+        ./manual-combined-pre.xml
+
+      ${pkgs.libxslt.bin}/bin/xsltproc \
+        -o manual-combined.xml ${./../../lib/make-options-doc/postprocess-option-descriptions.xsl} \
+        manual-combined-pre.xml
 
       ${linterFunctions}