docs: clarified release channel defaulting behavior for create cluster requests when release channel is unspecified

If release channel is left unspecified on cluster creation and a version is specified, the cluster is now enrolled in the most mature release channel where the version is available (first checking STABLE, then REGULAR, and finally RAPID) instead of being enrolled in STATIC. This only affects clusters with node auto upgrades enabled and no explicit node version specified. To ensure new clusters are enrolled in static, explicitly specify that the cluster should not be enrolled in a channel.

PiperOrigin-RevId: 532773421

google/container/v1/cluster_service.proto

@@ -1713,7 +1713,12 @@ message Cluster {
   // Shielded Nodes configuration.
   ShieldedNodes shielded_nodes = 40;
 
-  // Release channel configuration.
+  // Release channel configuration. If left unspecified on cluster creation and
+  // a version is specified, the cluster is enrolled in the most mature release
+  // channel where the version is available (first checking STABLE, then
+  // REGULAR, and finally RAPID). Otherwise, if no release channel
+  // configuration and no version is specified, the cluster is enrolled in the
+  // REGULAR channel with its default version.
   ReleaseChannel release_channel = 41;
 
   // Configuration for the use of Kubernetes Service Accounts in GCP IAM
@@ -2265,12 +2270,14 @@ message Operation {
   //
   // Examples:
   //
-  //   -
-  //   `https://container.googleapis.com/v1/projects/123/locations/us-central1/clusters/my-cluster`
-  //   -
-  //   `https://container.googleapis.com/v1/projects/123/zones/us-central1-c/clusters/my-cluster/nodePools/my-np`
-  //   -
-  //   `https://container.googleapis.com/v1/projects/123/zones/us-central1-c/clusters/my-cluster/nodePools/my-np/node/my-node`
+  // -
+  // ##
+  // `https://container.googleapis.com/v1/projects/123/locations/us-central1/clusters/my-cluster`
+  //
+  // ##
+  // `https://container.googleapis.com/v1/projects/123/zones/us-central1-c/clusters/my-cluster/nodePools/my-np`
+  //
+  // `https://container.googleapis.com/v1/projects/123/zones/us-central1-c/clusters/my-cluster/nodePools/my-np/node/my-node`
   string target_link = 7;
 
   // [Output only] The name of the Google Compute Engine