datastore: fix mutation error comment
datastore: fix mutation error comment

Describe clearly that NewInsert() will return gRPC AlreadyExists error
if given key already exists.

Fixes #1561

Change-Id: I94c5d97ca1d3981a8cd261bc272d2218e9677e41
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/49611
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Tyler Bui-Palsulich <b41e4aa57d2ad7eb1b29835a1b483e209dcc3c0e@google.com>

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