document why we retry in lambda source mapping

builtin/providers/aws/resource_aws_lambda_event_source_mapping.go

@@ -91,6 +91,13 @@ func resourceAwsLambdaEventSourceMappingCreate(d *schema.ResourceData, meta inte
 		Enabled:          aws.Bool(d.Get("enabled").(bool)),
 	}
 
+	// IAM profiles and roles can take some time to propagate in AWS:
+	//  http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/iam-roles-for-amazon-ec2.html#launch-instance-with-role-console
+	// Error creating Lambda function: InvalidParameterValueException: The
+	// function defined for the task cannot be assumed by Lambda.
+	//
+	// The role may exist, but the permissions may not have propagated, so we
+	// retry
 	err := resource.Retry(1*time.Minute, func() error {
 		eventSourceMappingConfiguration, err := conn.CreateEventSourceMapping(params)
 		if err != nil {