datastore: rename examples so they match the corresponding functions
datastore: rename examples so they match the corresponding functions

We want godoc to show the examples next to the functions or methods they
exemplify.

When there is a choice, we prefer functions over types, as with
Query/NewQuery.

Change-Id: I13f86c68e4c6762ad2699b3d9b0c83b102f06873
Reviewed-on: https://code-review.googlesource.com/7661
Reviewed-by: Jaana Burcu Dogan <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

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