I'm looking at 7.1/i386... Man page for man.conf(5) claims "_whatdb" to be the name of the apropos db. However, /etc/man.conf actually includes a reference to "_mandb".
ISTR _whatdb was working on 6.1 but haven't yet tested _whatdb vs. _mandb on a 7.1 system. Can someone more knowledgeable comment and indicate whether or not man page needs to be updated vs. the configuration file, itself?