docs: Describe the skip attribute for ValueEnums (#4328)

Documenting the `skip` attribute for the `ValueEnum` derive macro promptly, as it's not mentioned anywhere else.

Fixes #4327

src/_derive/mod.rs

@@ -258,6 +258,7 @@
 //!   - 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
 //!