Remove "Variant visibility" in `items/enumerations`

[GreenYun]

According to rust-lang/rust#28442 , visibility modifier for variant will lead to compilation error.

Consider removing the section or adding some notes about that.

[ehuss]

Enum variant visibility was added in rust-lang/rust#66183. The section Variant visibility explains that it is syntactically valid, but rejected during validation. Can you say more about why it appears wrong?