DOC: Note that URI normalization is experimental

Add a paragraph to the URI normalization documentation that URI normalization
is currently considered to be experimental.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 520db41..ce96c44 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -6019,6 +6019,11 @@
 
   Performs normalization of the request's URI.
 
+  URI normalization in HAProxy 2.4 is currently available as an experimental
+  technical preview. You should be prepared that the behavior of normalizers
+  might change to fix possible issues, possibly breaking proper request
+  processing in your infrastructure.
+
   Each normalizer handles a single type of normalization to allow for a
   fine-grained selection of the level of normalization that is appropriate for
   the supported backend.