I wrote:
"LDAP operations normally access only one database. However databases that provide related naming contexts ("suffixes") may be glued together with the /subordinate/ keyword. Some other features such as access controls and the /translucent/ overlay can also involve multiple databases."
I've added similar text to slapd.conf.5, but didn't see a suitable place for it in slapd-config.5. (Except in olcSubordinate description itself, where it is already mentioned).
Admin guide fix still needed.