Is the doc string for font-lock-add-keywords correct?

[Lennart Borgman (gmail)]

I just tested to add some keywords in the *scratch* buffer, but I can 
not get any changes to the fontification of the comment.

There was a discussion about the highlighting regular expressions 
before. hi-lock-mode seems to do a similar thing. I do not understand 
what it does, but does it not do a bit more to achieve the highlighting 
everywhere in the buffer? (hi-lock-mode works ok for me.)