GUACAMOLE-1935: Add note about different authentication mechanisms re…

…lated to VNC.

src/configuring-guacamole.md

@@ -485,6 +485,16 @@ as well as username and password authentication.
 : The password to use when attempting authentication, if any. This parameter
   is optional.
 
+These credentials may be requested by one of several different underlying
+authentication schemes, depending on the server configuration. The libvncclient
+library supports several of these, depending on the version of the library
+present on the system running guacd. Notably, the MSLogonII authentication
+protocol is only supported the most recent libvncclient release. If in
+doubt, or you encounter issues trying to log on to certain VNC servers,
+check the version of libvncclient that you're building guacd against and
+make sure it includes support for the authentication scheme of the
+server to which you're trying to connect.
+
 (vnc-display-settings)=
 
 #### Display settings