chore: clean up/improve code documentation for doc.deno.land

[crookse]

Summary

What:

View https://doc.deno.land/https://deno.land/x/rhum@v2.0.0/mod.ts. Make any necessary changes to the code to make it better (e.g., stub doc block needs the definition just like the other test doubles).

Why:

Make the API reference somewhat ok.