logging: clarify doc for CommonResource
logging: clarify doc for CommonResource

Addresses concerns in #598

Change-Id: Icca3ba036df388dc1b552f5e4f893c099b3e7722
Reviewed-on: https://code-review.googlesource.com/13670
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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