DOC: config: Explictly add supported MQTT versions

This avoids any ambiguities on supported versions. This patch depends on
129579813 ("MEDIUM: mqtt: support mqtt_is_valid and mqtt_field_value
converters for MQTTv3.1").

It must be backported with the above commit.

(cherry picked from commit 140a35766085bd900e8d9a74e3501abdb66d8a41)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit 9117829adc71de07d1d7fa96bc028e870e56d60f)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 0d4e64b..d844a1a 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -16604,6 +16604,8 @@
   CONNECT and CONNACK packet types. CONNECT is the first message sent by the
   client and CONNACK is the first response sent by the server.
 
+  Only MQTT 3.1, 3.1.1 and 5.0 are supported.
+
   Example:
 
       acl data_in_buffer req.len ge 4