tests/types: add suboptions doc test

Verify we are able to extract suboptions properly with our custom lib
extension.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

docs/default.nix

@@ -298,4 +298,7 @@ in
     in
     builtins.toJSON result.config.meta.maintainers
   );
+
+  # Unstable, for tests.
+  _internal = { inherit docsLib; };
 }