[Docs] Add 8.19 upgrade note: “Unchecked no-data checkbox now displays as Recover active alerts”

Customers upgrading from 8.17.x to 8.19.x may notice that existing Custom Threshold rules (and potentially Metric Threshold rules) show “Recover active alerts” selected under “If there is no data”, even though they never changed the rule.

This is a UI reinterpretation of legacy params, not a silent server-side migration. We should document this clearly in the 8.19 upgrade / release notes to avoid support escalations.

Raised on SDH.

## Details
In 8.19, no-data handling for Custom Threshold rules was unified into a single noDataBehavior field and radio group:

- [#254612](https://github.com/elastic/kibana/pull/254612) — backend support for noDataBehavior
- [#251976](https://github.com/elastic/kibana/pull/251976) — UI: replaced legacy checkboxes with three radio options

Previously (8.17 and earlier), users configured no-data behavior via:

- “Alert me if there is no data” (alertOnNoData)
- “Alert me if a group stops reporting data” (alertOnGroupDisappear) — for grouped rules

**Impact:**
Customer upgraded 8.17.x → 8.19.x
Rules that previously had the no-data checkbox unchecked now display “Recover active alerts”
Customer interpreted this as a silent behavior change during upgrade
During a data outage, many alerts fired; customer attributed this to the setting change
Important clarification for docs: “Recover active alerts” is the expected UI mapping for the old unchecked state. It does not mean the rule was changed to alert on missing data. Runtime behavior for rules with alertOnNoData: false and alertOnGroupDisappear: false should remain equivalent unless noDataBehavior is explicitly persisted.

## Proposed upgrade notes

Custom Threshold and Metric Threshold rules: “If there is no data” UI change
In 8.19, the “If there is no data” setting for Custom Threshold and Metric Threshold rules uses a unified set of options instead of separate checkboxes.

If you upgrade from 8.17.x or earlier, existing rules may display “Recover active alerts” even though you previously left the no-data checkbox unchecked. This is expected: the new UI maps your previous setting to the equivalent option.

Unchecked “Alert me if there is no data” on 8.17 → “Recover active alerts” on 8.19
Checked “Alert me if there is no data” on 8.17 → “Alert me about the missing data” on 8.19
Your rule parameters are not automatically changed during upgrade. The stored configuration remains the same until you edit and save the rule.

A new option, “Remain active” (do nothing), is also available in 8.19. This was not available on 8.17.

## Scope
- Custom Threshold rule (observability.rules.custom_threshold)
- Metric Threshold rule (same UI pattern; verify whether note should cover both)
- 8.19 release notes / upgrade guide (primary)
- Optionally backport note to 9.x docs if the upgrade path is still relevant

## Acceptance criteria
- [ ] 8.19 upgrade or release notes include a clear explanation of the UI mapping above
- [ ] Docs state that this is not a silent migration — params are unchanged until save
- [ ] Docs distinguish “Recover active alerts” from the new “Remain active” option
- [ ] Support can link to the doc when handling similar escalations

- Feature: [elastic/kibana#249850](https://github.com/elastic/kibana/issues/249850)
- UI PR: [elastic/kibana#251976](https://github.com/elastic/kibana/pull/251976)
- Backend PR: [elastic/kibana#254612](https://github.com/elastic/kibana/pull/254612)
- 8.19 backports: [#258364](https://github.com/elastic/kibana/pull/258364), [#258566](https://github.com/elastic/kibana/pull/258566)
- Metric threshold similar fix: [elastic/kibana#249662](https://github.com/elastic/kibana/pull/249662)