nixos/make-options-doc: treat missing descriptions as errors by default

this partially solves the problem of "missing description" warnings of the
options doc build being lost by nix build, at the cost of failing builds that
previously ran. an option to disable this behaviour is provided.

nixos/modules/misc/documentation.nix

@@ -15,7 +15,7 @@ let
       f = import m;
       instance = f (mapAttrs (n: _: abort "evaluating ${n} for `meta` failed") (functionArgs f));
     in
-      cfg.nixos.splitOptionDocBuild
+      cfg.nixos.options.splitBuild
         && builtins.isPath m
         && isFunction f
         && instance ? options
@@ -101,6 +101,7 @@ let
               exit 1
             } >&2
         '';
+    inherit (cfg.nixos.options) warningsAreErrors;
   };
 
 
@@ -256,7 +257,7 @@ in
         '';
       };
 
-      nixos.splitOptionDocBuild = mkOption {
+      nixos.options.splitBuild = mkOption {
         type = types.bool;
         default = true;
         description = ''
@@ -266,6 +267,15 @@ in
         '';
       };
 
+      nixos.options.warningsAreErrors = mkOption {
+        type = types.bool;
+        default = true;
+        description = ''
+          Treat warning emitted during the option documentation build (eg for missing option
+          descriptions) as errors.
+        '';
+      };
+
       nixos.includeAllModules = mkOption {
         type = types.bool;
         default = false;