Improve GeoIP2 struct documentation

Add comprehensive documentation for all GeoIP2 structs and fields:
- Document all top-level record types (Country, City, Enterprise, etc.)
- Add field descriptions with semantic meaning
- Include links to Wikipedia for ISO codes and standards
- Mark deprecated fields (metro_code)
- Document confidence scores for Enterprise fields
- Describe possible values for enum-like fields (connection_type, user_type)

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Co-Authored-By: Claude <noreply@anthropic.com>

Author: oschwald