This is the wiki page for the cvar
If you set this cvars value to anything but an empty string it overrides the Steam rich presence information with the value of the cvar. This text will be seen in the in your friends RL friendslist, as well as when your friends click on "View Game Info" in the Steam friendslist.
To turn on the detailed information, open the console and type
cl_online_status_override "custom text here"
If you put "", it will show the regular information again.