testing/protocmp: surface comment about unused Transform options
testing/protocmp: surface comment about unused Transform options

Change-Id: I83bdb0f3f937f745b5758c282e1bd58203a8c784
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/237221
Reviewed-by: Damien Neil <7e091e9a80da888fcd7836fd3c6b7b85f4677c12@google.com>

protocolbuffers/protobuf-go,protocolbuffers/protobuf-go