Correct all methods' signatures in Metamodel::EnumHOW's page

[Kaiepi]

None of its methods actually specify what type the object the method is
called on must be.

I just want to make sure this matches how the rest of the docs' signatures are written before pushing.

[Kaiepi]

If the object's type should be included despite the method not actually specifying it, this still needs correction anyway since its type would never be Metamodel::ClassHOW without a custom meta class that extends both or reparenting/reblessing/using .mixin on Metamodel::EnumHOW (why would you do that though?).

[JJ]

It matches current signature, so it's OK. Thanks.