Minor fixes to docs of UnaryExpression macro nodes

[HertzDevil]

• Indicates that Crystal::Macros::Splat is a subclass of Crystal::Macros::UnaryExpression. This is indeed the case for the underlying ASTNode types (Crystal::Splat < Crystal::UnaryExpression), from which the "subtyping" relationship in the macro language is derived.
• Documents Crystal::Macros::DoubleSplat. This already works in the language:

  macro foo(node)
    {% arg = node.args[0] %}
    {% arg %}                # => **x
    {% arg.class_name %}     # => "DoubleSplat"
    {% arg.exp %}            # => x
    {% arg.exp.class_name %} # => "Call"
  end
  
  foo(bar(**x))

• Removes a bit of code in the macro interpreter, since it is exactly the same as the superclass's definition (Crystal::UnaryExpression#interpret).

I left MacroVerbatim untouched. This is intentional because the rest of the macro nodes are undocumented.

[sdogruyol]

Thank you @HertzDevil 🙏

[straight-shoota]

Thank you @HertzDevil