logging: document per-request logging
logging: document per-request logging

Fixes #720.

Change-Id: I9dc35b72a25e783fa06de0b2d3a5cd22be96ead1
Reviewed-on: https://code-review.googlesource.com/31310
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

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