[DOC] add a few precisions about the use of RDP cookies

RDP cookies are not necessarily easy to implement because they require
some configuration on the servers. Add a few hints so that people know
what to check on their servers.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 096aac6..24cc3ac 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -3617,8 +3617,8 @@
                                  yes   |    no    |   yes  |   yes
   Arguments :
     <name>    is the optional name of the RDP cookie to check. If omitted, the
-              default cookie name "mstshash" will be used. There currently is
-              no valid reason to change this name.
+              default cookie name "msts" will be used. There currently is no
+              valid reason to change this name.
 
   This statement enables persistence based on an RDP cookie. The RDP cookie
   contains all information required to find the server in the list of known
@@ -3633,6 +3633,10 @@
   load-balancing method. Thus it is highly recommended to put all statements in
   a single "listen" section.
 
+  Also, it is important to understand that the terminal server will emit this
+  RDP cookie only if it is configured for "token redirection mode", which means
+  that the "IP address redirection" option is disabled.
+
   Example :
         listen tse-farm
             bind :3389