nixos/documentation: deprecate docbook option docs

following the plan in https://github.com/NixOS/nixpkgs/pull/189318#discussion_r961764451

also adds an activation script to print the warning during activation
instead of during build, otherwise folks using the new CLI that hides
build logs by default might never see the warning.

nixos/lib/testing/testScript.nix

@@ -7,7 +7,7 @@ in
   options = {
     testScript = mkOption {
       type = either str (functionTo str);
-      description = ''
+      description = mdDoc ''
         A series of python declarations and statements that you write to perform
         the test.
       '';