add note in migration guide

docs/content/reference/migration/migration-0-22.md

@@ -1,9 +1,9 @@
 ---
-title: Migrating from 0.20 to 0.21
-order: 989
+title: Migrating from 0.21 to 0.22
+order: 988
 ---
 
-### Previously deprecated `DisconnectedSpace` archetype/component got now removed.
+### Previously deprecated `DisconnectedSpace` archetype/component got now removed
 
 The deprecated `DisconnectedSpace` archetype and `DisconnectedSpace` component have been removed.
 To achieve the same effect, you can log any of the following "invalid" transforms:
@@ -16,3 +16,12 @@ Previously, the `DisconnectedSpace` archetype played a double role by governing
 This led to a lot of complexity and often broke or caused confusion (see https://github.com/rerun-io/rerun/issues/6817, https://github.com/rerun-io/rerun/issues/4465, https://github.com/rerun-io/rerun/issues/4221).
 By now, explicit blueprints offer a better way to express which views should be spawned and what content they should query.
 (you can learn more about blueprints [here](https://rerun.io/docs/getting-started/configure-the-viewer/through-code-tutorial)).
+
+
+### Removed `num_instances` keyword argument to `rr.log_components()`
+
+For historical reasons, the `rr.log_components()` function of the Python SDK accepts an optional, keyword-ony argument `num_instances`.
+It was no longer used for several releases, so we removed it.
+
+**Note**: although `rr.log_components()` is technically a public API, it is undocumented, and we discourage using it.
+For logging custom components, use [`rr.AnyValue`](https://ref.rerun.io/docs/python/main/common/custom_data/#rerun.AnyValues) and [`rr.AnyBatchValue`](https://ref.rerun.io/docs/python/main/common/custom_data/#rerun.AnyBatchValue).