DOC: quic: specify that connection migration is not supported

Currently haproxy does not support QUIC connection migration. This is
advertized to clients on their connections. Document this in the first
QUIC related paragraph.

This should be backported up to 2.6.

(cherry picked from commit 59b69aafae0db672509a7469cc59cc2e87cc0b32)
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
(cherry picked from commit 939f4176dc5c53f629af0890fe6e4eb0eeed0e90)
 [ad: context adjustment]
Signed-off-by: Amaury Denoyelle <adenoyelle@haproxy.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 5b5ad1b..b19899a 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -201,7 +201,8 @@
 HTTP/3 is implemented over QUIC, itself implemented over UDP. QUIC solves the
 head of line blocking at transport level by means of independently treated
 streams. Indeed, when experiencing loss, an impacted stream does not affect the
-other streams.
+other streams. QUIC also provides connection migration support but currently
+haproxy does not support it.
 
 By default HAProxy operates in keep-alive mode with regards to persistent
 connections: for each connection it processes each request and response, and