Clarify we never receive CHANNEL_OPEN_FAILURE

lib/msf/core/exploit/ssh/auth_methods.rb

@@ -179,6 +179,9 @@ module Msf::Exploit::Remote::SSH::AuthMethods
       # channel open, since most implementations I've seen support only one
       # session channel and don't support channel closing, so this would block
       # us from getting a shell
+      #
+      # Secondly, libssh doesn't send a CHANNEL_OPEN_FAILURE when we're not
+      # authed, so we have to wait for a timeout on CHANNEL_OPEN to return false
 
       # So assume we succeeded until we can verify
       true