Update ZDT update limitation to only bulkUpdate (#167200)

## Summary

We've fixed the update limitation
(https://github.com/elastic/kibana/issues/152807), so now it only
applies to bulkUpdates https://github.com/elastic/kibana/issues/165434

### Checklist

Delete any items that are not applicable to this PR.

- [ ] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [ ]
[Documentation](https://www.elastic.co/guide/en/kibana/master/development-documentation.html)
was added for features that require explanation or tutorials
- [ ] [Unit or functional
tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html)
were updated or added to match the most common scenarios
- [ ] Any UI touched in this PR is usable by keyboard only (learn more
about [keyboard accessibility](https://webaim.org/techniques/keyboard/))
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### Risk Matrix

Delete this section if it is not applicable to this PR.

Before closing this PR, invite QA, stakeholders, and other developers to
identify risks that should be tested prior to the change/feature
release.

When forming the risk matrix, consider some of the following examples
and how they may potentially impact the change:

| Risk | Probability | Severity | Mitigation/Notes |

|---------------------------|-------------|----------|-------------------------|
| Multiple Spaces—unexpected behavior in non-default Kibana Space.
| Low | High | Integration tests will verify that all features are still
supported in non-default Kibana Space and when user switches between
spaces. |
| Multiple nodes—Elasticsearch polling might have race conditions
when multiple Kibana nodes are polling for the same tasks. | High | Low
| Tasks are idempotent, so executing them multiple times will not result
in logical error, but will degrade performance. To test for this case we
add plenty of unit tests around this logic and document manual testing
procedure. |
| Code should gracefully handle cases when feature X or plugin Y are
disabled. | Medium | High | Unit tests will verify that any feature flag
or plugin combination still results in our service operational. |
| [See more potential risk
examples](https://github.com/elastic/kibana/blob/main/RISK_MATRIX.mdx) |


### For maintainers

- [ ] This was checked for breaking API changes and was [labeled
appropriately](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

packages/core/saved-objects/core-saved-objects-server/docs/model_versions.md

@@ -893,9 +893,11 @@ to the `fields` option **were already present in the prior model version**. Othe
 during upgrades, where newly introduced or backfilled fields may not necessarily appear in the documents returned
 from the `search` API when the option is used.
 
-### Using `update` with dynamically backfilled fields
+### Using `bulkUpdate` with dynamically backfilled fields
 
-The savedObjects `update` API is effectively a partial update (using Elasticsearch's `_update` under the hood),
+(Note: this same limitation used to exist for the `update` method but has been [fixed](https://github.com/elastic/kibana/issues/165434). So while they're similar this limitation is only relevant for the `bulkUpdate` method)
+
+The savedObjects `bulkUpdate` API is effectively a partial update (using Elasticsearch's `_update` under the hood),
 allowing API consumers to only specify the subset of fields they want to update to new values, without having to
 provide the full list of attributes (the unchanged ones). We're also not changing the `version` of the document
 during updates, even when the instance performing the operation doesn't know about the current model version
@@ -935,8 +937,14 @@ const newDocAttributes = {
 Which could occur either while being still in the cohabitation period, or in case of rollback:
 
 ```ts
-savedObjectClient.update('type', 'id', {
-  index: 11,
+savedObjectClient.bulkUpdate({
+  objects: [{
+    type: 'type', 
+    id: 'id', 
+    attributes: {
+      index: 11
+    }
+  }]
 });
 ```
 
@@ -949,7 +957,7 @@ We will then be in a situation where our data is **inconsistent**, as the value
 }
 ```
 
-The long term solution for that is implementing [backward-compatible updates](https://github.com/elastic/kibana/issues/152807), however
+The long term solution for that is implementing [backward-compatible updates](https://github.com/elastic/kibana/issues/165434), however
 this won't be done for the MVP, so the workaround for now is to avoid situations where this edge case can occur.
 
 It can be avoided by either: