firestore: document that a ts.Timestamp can be used when saving data.
firestore: document that a ts.Timestamp can be used when saving data.

Change-Id: I12345790f2e6f73427a46e9c1a614678358bc4b4
Reviewed-on: https://code-review.googlesource.com/27170
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go