docs: Clarify Batch only supports global custom instance template now (…

…#27390)

google-cloud-batch-v1/proto_docs/google/cloud/batch/v1/job.rb

@@ -515,7 +515,10 @@ class InstancePolicy
  # @return [::String]
  # Name of an instance template used to create VMs.
  # Named the field as 'instance_template' instead of 'template' to avoid
- # c++ keyword conflict.
+ # C++ keyword conflict.
+ #
+ # Batch only supports global instance templates.
+ # You can specify the global instance template as a full or partial URL.
  # @!attribute [rw] install_gpu_drivers
  # @return [::Boolean]
  # Set this field true if you want Batch to help fetch drivers from a third