[Security Center] Update Brand Protection domain search

[decompi]

Summary

Updates the Brand Protection documentation to match the current dashboard workflow:

• Documents the Add string and Save query flow.
• Clarifies that each string is matched as a complete phrase.
• Documents automatic background lookback.
• Removes the legacy character-distance workflow.

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit e3e373d.

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