config/session-management: add pam_rundir to xdg_runtime_dir

[cyborgcatboy]

this pr adds a paragraph underneath XDG_RUNTIME_DIR that describes how to use pam_rundir to set it up, as this in my mind is more helpful than just telling users to install elogind or to set it up manually (just adding an alternative instruction) ive tried to keep it as close to the guidelines as possible, but i may have missed something >w< so any feedback would be appreciated meow !!! :3