diff --git a/doc/configuration.txt b/doc/configuration.txt
index 90a0346..51f2964 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -2984,7 +2984,8 @@
     - "set-header" does the same as "add-header" except that the header name
       is first removed if it existed. This is useful when passing security
       information to the server, where the header must not be manipulated by
-      external users.
+      external users. Note that the new value is computed before the removal so
+      it is possible to concatenate a value to an existing header.
 
     - "del-header" removes all HTTP header fields whose name is specified in
       <name>.