[Cache Rules] Document "is in list" and "is not in list" operators in hostname match expressions

[jayeve]

Document the "is in list" and "is not in list" operators for Cache Rules. These operators allow Enterprise customers to match requests against custom hostname lists in cache rule expressions — for example, applying a caching configuration to all hostnames in a managed list rather than enumerating them individually.

Changes:

• settings.mdx: Add "is in list" / "is not in list" to the available operators list, with a note explaining they are available for the Hostname field on Enterprise plans and linking to the custom lists documentation.
• New example (cache-by-hostname-list.mdx): Add an example showing how to create a cache rule that caches everything for hostnames matching a custom hostname list, with both Expression Builder and Expression Editor syntax.

Context:

• The corresponding dashboard change exposes the "is in list" operator in the cache rules expression builder for Enterprise zones.
• Note: The custom lists documentation currently lives under /waf/tools/lists/ despite being an account-level, product-agnostic feature used by 11+ products (WAF, DDoS, Network Firewall, Cloudflare One, etc.). A future effort could relocate it to a more neutral home like /rules/ or /ruleset-engine/.

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.