Add commit timestamp documentation for Spanner
Add commit timestamp documentation for Spanner

This adds in a sample we already had (but weren't using) and
includes a caveat about DML.

Fixes #2975 as far as we can in this repo.

jskeet/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/google-cloud-dotnet,jskeet/gcloud-dotnet,googleapis/google-cloud-dotnet,googleapis/google-cloud-dotnet,jskeet/google-cloud-dotnet