vendor: update all dependencies

vendor/cloud.google.com/go/firestore/doc.go

@@ -21,6 +21,9 @@ database.
 See https://cloud.google.com/firestore/docs for an introduction
 to Cloud Firestore and additional help on using the Firestore API.
 
+See https://godoc.org/cloud.google.com/go for authentication, timeouts,
+connection pooling and similar aspects of this package.
+
 Note: you can't use both Cloud Firestore and Cloud Datastore in the same
 project.
 
@@ -209,10 +212,5 @@ read and write methods of the Transaction passed to it.
 	if err != nil {
 		// TODO: Handle error.
 	}
-
-Authentication
-
-See examples of authorization and authentication at
-https://godoc.org/cloud.google.com/go#pkg-examples.
 */
 package firestore