docs: Describe the skip attribute for ValueEnums
Documenting the `skip` attribute for the `ValueEnum` derive macro promptly, as it's not mentioned anywhere else.
Fixes
//! - When not present: case-converted field name is used
//! - `help = <expr>`: [`PossibleValue::help`][crate::builder::PossibleValue::help]
//! - When not present: [Doc comment summary](#doc-comments)
+//! - `skip`: Ignore this variant
//!
//! ## Arg Types
//!
