jsondoclint: simplify code using idiomatic Rust

[AMMAAR-IC]

Simplify jsondoclint with small idiomatic Rust cleanups. No behavior changes.

• Use matches!() macro instead of match in item_kind.rs
• Remove unnecessary derefs and simplify Option mapping in validator.rs
• Replace clone() on Copy type with *id
• Fix doc comment indentation
• Remove redundant tuple parens in tests.rs
• Remove needless borrow and closure in main.rs

Verified:

• cargo clippy -p jsondoclint --tests --quiet → no warnings
• cargo test -p jsondoclint --quiet → all 5 tests pass

[rustbot]

r? @Mark-Simulacrum

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• Fallback group: @Mark-Simulacrum, @jieyouxu
• @Mark-Simulacrum, @jieyouxu expanded to Mark-Simulacrum, jieyouxu