add note for a feature gate

Author: sanposhiho

content/en/docs/concepts/scheduling-eviction/topology-spread-constraints.md

@@ -96,6 +96,14 @@ your cluster. Those fields are:
   A domain is a particular instance of a topology. An eligible domain is a domain whose
   nodes match the node selector.
 
+  <!-- OK to remove this note once v1.29 Kubernetes is out of support -->
+  {{< note >}}
+  Before Kubernetes v1.30, the `minDomains` field was only available if the
+  `MinDomainsInPodTopologySpread` [feature gate](/docs/reference/command-line-tools-reference/feature-gates/)
+  was enabled (default since v1.28). In older Kubernetes clusters it might be explicitly
+  disabled or the field might not be available.
+  {{< /note >}}
+
   - The value of `minDomains` must be greater than 0, when specified.
     You can only specify `minDomains` in conjunction with `whenUnsatisfiable: DoNotSchedule`.
   - When the number of eligible domains with match topology keys is less than `minDomains`,