Document when ssl_prefer_server_ciphers went in

In the documentation for ssl_prefer_server_ciphers we only say it's not in
"older version" but we omit to specify it further.  Since it's a fairly
important setting for security I think it makes sense to add the version to
help users, as in the small attached diff (which also adds proper markup in the
paragraph while in there).

--
Daniel Gustafsson