MINOR: doc: Document the interaction of allow-0rtt and retry-on 0rtt-rejected.

Document that haproxy will try to use early data when connecting to the
server if we have retry-on 0rtt-rejected, as well.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 674134e..95f345e 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -11756,7 +11756,8 @@
 
 allow-0rtt
   Allow sending early data to the server when using TLS 1.3.
-  Note that early data will be sent only if the client used early data.
+  Note that early data will be sent only if the client used early data, or
+  if the backend uses "retry-on" with the "0rtt-rejected" keyword.
 
 alpn <protocols>
   This enables the TLS ALPN extension and advertises the specified protocol