docs(cli): expand long help

[maksym-arutyunyan]

Summary

• explain when # Keep sorted comment is required
• document skip keywords and comment types
• show return codes in a small table

Testing

• cargo build --release --all-targets
• cargo test
• cargo test --release
• cargo clippy --all-targets -- -D warnings
• cargo fmt --all -- --check
• git ls-files -z | grep -vzE '^tests/|^e2e-tests/|^README.md$' | xargs -0 -n1 ./target/release/keepsorted --features gitignore,rust_derive_canonical
• bats e2e-tests
• ./run-all.sh

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https://chatgpt.com/codex/tasks/task_e_6883abc9ae80832d85bcceaa56e01788