Add update_strategy deprecation message to 1.20.0. (#2575)

Part of #2551

Author: rileykarson

google/resource_compute_region_instance_group_manager.go

@@ -145,6 +145,7 @@ func resourceComputeRegionInstanceGroupManager() *schema.Resource {
 				Type:         schema.TypeString,
 				Optional:     true,
 				Default:      "NONE",
+				Deprecated:   "This field will have no functionality in 2.0.0, and will be removed. If you're using ROLLING_UPDATE, use the google-beta provider. See https://terraform.io/docs/providers/google/provider_versions.html for more details.",
 				ValidateFunc: validation.StringInSlice([]string{"NONE", "ROLLING_UPDATE"}, false),
 			},
 

website/docs/r/compute_region_instance_group_manager.html.markdown

@@ -124,7 +124,9 @@ The following arguments are supported:
     resource is modified, a value of `"NONE"` will prevent any of the managed
     instances from being restarted by Terraform. A value of `"ROLLING_UPDATE"`
     is supported as a beta feature. A value of `"ROLLING_UPDATE"` requires
-    `rolling_update_policy` block to be set. 
+    `rolling_update_policy` block to be set. This field is deprecated as in
+    `2.0.0` it has no functionality anymore. It will be removed then. This field
+    is only present in the `google` provider.
 
 * `target_size` - (Optional) The target number of running instances for this managed
     instance group. This value should always be explicitly set unless this resource is attached to