[Cache] Document selected header matching

[foreseaz]

Summary

Clarifies that selected values for custom and restricted cache-key headers use case-sensitive substring matching. Includes examples of a comma-separated header value, a longer value, and a case difference.

Documentation checklist

• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 6982d5b.

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