Add testing note about session channel opens

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

@@ -175,6 +175,11 @@ module Msf::Exploit::Remote::SSH::AuthMethods
         :byte, USERAUTH_SUCCESS
       ))
 
+      # We can't fingerprint or otherwise reduce false positives using a session
+      # 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
+
       # So assume we succeeded until we can verify
       true
     end