Document SLO transform limitations related to misreported state and P… (#3123)

This PR adds a new limitation to the [Transforms
limitation](https://www.elastic.co/docs/explore-analyze/transforms/transform-limitations)
page about the behavior of SLO transforms at scale. When many SLO
transforms run concurrently, two issues can occur:

- Transforms may misreport their state and suggest deletion, even though
they remain valid.
- PIT requests may accumulate and create excess activity on the cluster,
leading to degraded performance.

The new section explains these limitations.

---------

Co-authored-by: István Zoltán Szabó <[email protected]>

Author: kosabogi

explore-analyze/transforms/transform-limitations.md

@@ -123,6 +123,55 @@ If your data uses the [date nanosecond data type](elasticsearch://reference/elas
 
 If you use ILM to have time-based indices, consider using the [Date index name](elasticsearch://reference/enrich-processor/date-index-name-processor.md) instead. The processor works without duplicated documents if your {{transform}} contains a `group_by` based on `date_histogram`.
 
+### SLO transforms at scale may misreport state and increase PIT activity
+
+When running a large number of SLO {{transforms}}, two types of limitations can appear:
+
+- {{transforms-cap}} may [misreport](#transforms-inaccurate-errors) their state and suggest deletion even though they are still valid
+- Point-in-time (PIT) requests may [accumulate](#transforms-pit-storms), creating extra activity on the cluster and slowing down operations
+
+#### {{transforms-cap}} can return inaccurate errors that suggest deletion [transforms-inaccurate-errors]
+
+The {{transforms-cap}} API and the {{transforms-cap}} page in {{kib}} (**Stack Management** > **{{transforms-cap}})** may display misleading error messages for {{transforms}} created by service level objectives (SLOs).
+
+The message typically reads:
+
+```console
+Found task for transform [...], but no configuration for it. To delete this transform use DELETE with force=true.
+```
+Although the error suggests that the {{transform}} is broken and should be deleted, this is usually not the case. The error occurs because of a limitation in how the {{transforms}} are queried and presented by the API and UI. Deleting the {{transform}} can break related SLOs.
+
+If you encounter this error, do not delete the {{transform}}. If a {{transform}} was already deleted, you can restore it by resetting the SLO, either in the {{kib}} or with the API:
+
+- In {{kib}}, go to the **Observability** > **SLOs application**, open the menu for the affected SLO, and click **Reset**. 
+
+- If you use the API, run the following request:
+
+  ```console
+  POST kbn:/api/observability/slos/<SLO_ID>/_reset
+  ```
+
+#### Large numbers of {{transform}} can cause PIT overloads [transforms-pit-overloads]
+
+{{transforms-cap}} rely on point-in-time (PIT) searches to ensure that queries remain consistent during data changes. Each {{transform}} can open and close multiple PITs during its lifetime. 
+
+When many {{transforms}} run concurrently, especially in environments with large numbers of SLOs (hundreds to more than a thousand transforms), PITs can be opened and closed in quick succession. Because PITs are closed asynchronously, the close operation does not wait for the previous request to complete. This can create a backlog of PIT close requests, known as a PIT overload.
+
+A PIT overload increases the memory pressure on hot nodes. This may result in rising memory usage and long garbage collection cycles. In practice, searches and writes may slow down or queue until the node recovers, and in severe cases a restart may be required.
+
+As a workaround, you can disable PIT for a specific {{transform}}:
+
+```console
+POST _transform/<transform_id>/_update
+{
+  "settings": {
+    "use_point_in_time": false
+  }
+}
+```
+
+To fix this issue permanently, upgrade to the following versions or later: 8.18.8, 8.19.5, 9.0.8, 9.1.5, or 9.2.0.
+
 ## Limitations in {{kib}} [transform-ui-limitations]
 
 ### {{transforms-cap}} are visible in all {{kib}} spaces [transform-space-limitations]