You aren't likely to find documentation on the folders themselves. You can find documentation on the specific files though.
$DLC/properties contains the various config files for the name server, appserver, webspeed and other *man (dbman,asbman,etc) utilities as well as the adminserver plugins.
$DLC/certs would contain any SSL keys.
In my opinion you should back up the entire $DLC directory... so easy to restore/replicate instead of reinstalling Progress again.