datastore: mark iterator unsafe for concurrent use

Fixes #1535

Change-Id: Icb44598adf4405943c3ec038997cbb37e024116d
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/44130
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
diff --git a/datastore/query.go b/datastore/query.go
index 5851662..f2e6d9c 100644
--- a/datastore/query.go
+++ b/datastore/query.go
@@ -594,6 +594,8 @@
 }
 
 // Iterator is the result of running a query.
+//
+// It is not safe for concurrent use.
 type Iterator struct {
 	ctx    context.Context
 	client *Client