[datastore] improve docs around auto-generated *Keys on Put.

service/datastore/key.go

Index: service/datastore/key.go
diff --git a/service/datastore/key.go b/service/datastore/key.go
index 8757f61ec57c318ad2ea0af65c6beadfd0e53cc1..5dc34378e688219ffaccd3d1bcb0dd55d3592732 100644
--- a/service/datastore/key.go
+++ b/service/datastore/key.go
@@ -250,7 +250,8 @@ func (k *Key) String() string {
 	return b.String()
 }
 
-// IsIncomplete returns true iff k doesn't have an id yet.
+// IsIncomplete returns true iff the last token of this Key doesn't define
+// either a StringID or an IntID.
 func (k *Key) IsIncomplete() bool {
 	return k.LastTok().IsIncomplete()
 }