Clarify ignore-codes CLI documentation

[ekamran]

Summary

Clarifies how --ignore-codes works in the WP-CLI documentation.

Changes

• Explains that result codes are different from check slugs.
• Adds an example for discovering result codes with --format=csv --fields=code,message.
• Adds an example for ignoring missing_composer_json_file.
• Clarifies that --exclude-files and --exclude-directories only apply to file-based scans and do not suppress plugin-level findings.

Fixes #1152.

[github-actions]

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Co-authored-by: ekamran <ekamran@git.wordpress.org>
Co-authored-by: ernilambar <nilambar@git.wordpress.org>
Co-authored-by: marekdedic <marekdedic@git.wordpress.org>

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