docs: Tighten up some documentation for g:c:spanner::Client::Commit() (googleapis/google-cloud-cpp-spanner#1173)
docs: Tighten up some documentation for g:c:spanner::Client::Commit() (googleapis/google-cloud-cpp-spanner#1173)


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