Document mux sockets as a potential issue

[ryancdotorg]

Hi,

This isn't a bug or a vulnerability, but I think your documentation should call out that access control could be subverted by malicious software on an endpoint enabling muxing (e.g. using the ControlMaster option in ~/.ssh/config) to hold open a connection and/or open additional sessions without further authentication.

It seems that this can be addressed server-side by setting MaxSessions 1 in the server config file.