IN BOOLEAN MODE does not overcome 50% limit as claimed

Experimenting with FULLTEXT, it took me awhile to make a match across columns work at all because the examples in the documentation are incomplete and I didn't know how to set up to use that kind of matching (I'm not even sure what the FULLTEXT keyword's technical role is, since it doesn't appear in the output of DESCRIBE).

Because I need to match keywords that may appear in more than half the records, I was matching IN BOOLEAN MODE. But I was getting "empty set" back. So I tried matching on a rare term and, of course, got a hit back. Clearly, the docs are wrong or (I hope) there's a bug in the IN BOOLEAN MODE qualifier.