Document generated defaults in config reference

[MukundaKatta]

Summary

• add generated-reference helpers for section-specific defaults
• show resticprofile defaults for retention path, retention tag, and related host rows
• cover the new reference default helper in template tests

Closes #200

Verification

• go test ./config
• go run . generate --config-reference --to /tmp/resticprofile-reference-fixed
• git diff --check

Note: go test ./... was attempted first, but this local checkout is missing helper binaries under build/ such as test-shell, test-crontab, and test-args.