DOC: cache: Add new caching limitation information

Responses that do not have an explicit expiration time or a validator
will not be cached anymore.

Must be backported if cc9bf2e ("MEDIUM: cache: Change caching
conditions") is backported.

(cherry picked from commit d493bc863d85182b663d4a8869ec805d74e712e8)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 70e0151..838cf0e 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -14283,6 +14283,9 @@
 - 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