DOC: management: mark "set server ssl" as deprecated

This command was integrated in 2.4 when it was not possible to handle
SSL with dynamic servers, this is now possible so we should prefer this
way.

Must be backported in 2.5.
diff --git a/doc/management.txt b/doc/management.txt
index b96cbc8..b9b7ebd 100644
--- a/doc/management.txt
+++ b/doc/management.txt
@@ -2185,11 +2185,14 @@
   Change a server's FQDN to the value passed in argument. This requires the
   internal run-time DNS resolver to be configured and enabled for this server.
 
-set server <backend>/<server> ssl [ on | off ]
+set server <backend>/<server> ssl [ on | off ]  (deprecated)
   This option configures SSL ciphering on outgoing connections to the server.
   When switch off, all traffic becomes plain text; health check path is not
   changed.
 
+  This command is deprecated, create a new server dynamically with or without
+  SSL instead, using the "add server" command.
+
 set severity-output [ none | number | string ]
   Change the severity output format of the stats socket connected to for the
   duration of the current session.