Docs: reminder — cursor ack state persistence tuning guide

## Reminder

During the review of apache/pulsar#25548, @lhotari raised the need for a tuning guide alongside the new WARN log and metrics:

> Adding documentation for the tuning together with the warning log in code will help. [...] Please go ahead with the warning log (and any documentation improvements).
> — https://github.com/apache/pulsar/issues/25540#issuecomment-4261938708

The related doc PR apache/pulsar-site#1118 only covers the two new OpenTelemetry counters. The broader tuning guide hasn't been written.

Opening this as a reminder so it doesn't get lost. Happy to pass it on to anyone with the right context on the project to pick it up.

## References

- Original issue: apache/pulsar#25540
- Code PR: apache/pulsar#25548
- Metric doc PR: apache/pulsar-site#1118
- lhotari's tuning input: https://github.com/apache/pulsar/issues/25540#issuecomment-4261938708

Start by reading the original issue apache/pulsar#25540, the code PR apache/pulsar#25548, and the related documentation PR apache/pulsar-site#1118. Done means adding the broader cursor acknowledgement-state persistence tuning guide alongside the WARN log and metrics documentation.

Written by the indexing model from the issue text.