docs: document missing queue item error

[lsaether]

Summary

• document the dedicated -32004 JSON-RPC error for stale/missing queue items in rooms/unqueue_prompt
• clarify that clients can distinguish missing queue state from malformed params

This completes the docs half of the queue-item-not-found follow-up from #43; the implementation/test are already present on main after the rooms-namespace refactor.

Refs #43.

Verification

• git diff --check origin/main...HEAD
• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• added-line security/private-network scan: no findings