Update documentation of BulkImportRelationshipsRequest

Author: tstirrat15

authzed/api/v1/permission_service.proto

@@ -670,7 +670,9 @@ message ResolvedSubject {
 // ImportBulkRelationshipsRequest represents one batch of the streaming
 // ImportBulkRelationships API. The maximum size is only limited by the backing
 // datastore, and optimal size should be determined by the calling client
-// experimentally.
+// experimentally. Any relationships within the same request are guaranteed to
+// be written in a single transaction. If any of the relationships already
+// exist, the transaction will fail and no relationships will be written.
 message ImportBulkRelationshipsRequest {
   repeated Relationship relationships = 1
       [ (validate.rules).repeated .items.message.required = true ];