doc: implement the spanner_dml_batch_update example (googleapis/google-cloud-cpp-spanner#1081)
doc: implement the spanner_dml_batch_update example (googleapis/google-cloud-cpp-spanner#1081)

* doc: implement the spanner_dml_batch_update example

* update the comment

* Use Commit

* really resolved the conflict

* address code review comments

googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp,googleapis/google-cloud-cpp