* lisp/net/tls.el (tls-program): Document the %t parameter (bug#22559).
"openssl s_client -connect %h:%p -no_ssl2 -ign_eof")
"List of strings containing commands to start TLS stream to a host.
Each entry in the list is tried until a connection is successful.
-%h is replaced with server hostname, %p with port to connect to.
+%h is replaced with the server hostname, %p with the port to
+connect to, and %t with a file name containing trusted certificates.
The program should read input on stdin and write output to stdout.
See `tls-checktrust' on how to check trusted root certs.