docs(ko): fix splat parameter type annotations

- Single splat (*args): change from `*args: Type` to `*args: Array<Type>`
  since Ruby collects rest arguments into an Array
- Double splat (**kwargs): change from `**kwargs: Type` to
  `**kwargs: Hash<Symbol, Type>` since Ruby collects keyword rest
  arguments into a Hash
- Update summary table and explanation text accordingly
- Add new keyword argument syntax with `**{ }` for inline types
- Add interface reference syntax for keyword arguments

Author: yhk1038