diff --git a/doc/configuration.txt b/doc/configuration.txt
index ee93b39..ccc116e 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -14300,6 +14300,9 @@
 - If the response contains a Vary header
 - If the Content-Length + the headers size is greater than "max-object-size"
 - If the response is not cacheable
+- If the response does not have an explicit expiration time (s-maxage or max-age
+  Cache-Control directives or Expires header) or a validator (ETag or Last-Modified
+  headers)
 
 - If the request is not a GET
 - If the HTTP version of the request is smaller than 1.1