User Task Listeners update guide changes (#4804)

* docs: describe changed user task events

This is relevant to users updating from 8.6 to 8.7 that rely on the
`ASSIGNING` record from the exporters specifically. Either because they
build their own exporter, or because they use the data in ES/OS or some
other external data storage that has received this information.

* docs: describe assignee not in created events

This is relevant to anyone depending on the `assignee` in the exported
`USER_TASK` records `CREATING/CREATED`.

Co-authored-by: Dmitriy Melnychuk <[email protected]>
Co-authored-by: Ana <[email protected]>

docs/self-managed/operational-guides/update-guide/860-to-870.md

@@ -12,3 +12,50 @@ The following sections explain which adjustments must be made to migrate from Ca
 
 Configuring a non-existing bucket for backups will not prevent Zeebe to start up anymore and will only result
 in logs (at WARN) in the startup phase.
+
+## Exported records
+
+### `USER_TASK` records
+
+To support User Task Listeners, some backward incompatible changes were necessary to the exported `USER_TASK` records.
+
+#### `assignee` no longer provided in `CREATING/CREATED` events
+
+Previously, when a user task was activating with a specified `assignee`,
+we appended the following events of the `USER_TASK` value type:
+
+- `CREATING` with `assignee` property as provided
+- `CREATED` with `assignee` property as provided
+
+The `ASSIGNING` and `ASSIGNED` events were not appended in this case.
+
+To support the new User Task Listeners feature, the `assignee` value will not be filled in the `CREATING` and `CREATED` events anymore.
+
+With 8.7, the following events are now appended:
+
+- `CREATING` with `assignee` always `""` (empty string)
+- `CREATED` with `assignee` always `""` (empty string)
+- `ASSIGNING` with `assignee` property as provided
+- `ASSIGNED` with `assignee` property as provided
+
+#### `ASSIGNING` has become `CLAIMING` for `CLAIM` operation
+
+When claiming a user task, we previously appended the following records of the `USER_TASK` value type:
+
+- `CLAIM`
+- `ASSIGNING`
+- `ASSIGNED`
+
+A new `CLAIMING` intent was introduced to distinquish between claiming and regular assigning.
+We now append the following records when claiming a user task:
+
+- `CLAIM`
+- `CLAIMING`
+- `ASSIGNED`
+
+The `ASSIGNING` event is still appended for assigning a user task.
+In that case, we append the following records:
+
+- `ASSIGN`
+- `ASSIGNING`
+- `ASSIGNED`