datastore: add/improve doc on Key methods.
datastore: add/improve doc on Key methods.

Change-Id: I894c091d910d63e2a29f37b8bf8e803dd52ce5da
Reviewed-on: https://code-review.googlesource.com/7692
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go