Added note

static/defaults.config.php

@@ -617,6 +617,7 @@ return [
 	'performance' => [
 		// max_response_data_size (Integer)
 		// Maximum allowed outgoing HTTP request response data size in Bytes. Does not affect incoming requests to this node.
+		// Warning: Lowering this value can help with some PHP memory exhaustion issues, but can also partially break some federation features e.g. large posts may not be fetched or received from remote servers.
 		'max_response_data_size' => 1000000,
 	],
 	'proxy' => [