Just say that the databases are intended for use by apropos(1), whatis(1),

Just say that the databases are intended for use by apropos(1), whatis(1),
and man(1), without restricting that statement to "man -k".
Suggested by and patch OK'ed by jmc@.

While only apropos(1) and whatis(1) strictly require the database
and while our man(1) implementation can find many manual pages even
when no database is available or when the database is incomplete or
corrupt, it does use the database even without -k whenever possible.
Consequently, this change makes the manual page less confusing.