[Session Plugin] update man page after adding new config options
[Session Plugin] update man page after adding new config options

Add descriptions for the config options `cookie_http_only` and
`cookie_secure` to the plugin's man page.

cutelyst/cutelyst,buschmann23/cutelyst,cutelyst/cutelyst,cutelyst/cutelyst,buschmann23/cutelyst,buschmann23/cutelyst