Fix retain comment to make more sense

[akiramusic000]

This changes the retain doc comment from:
Note that this clones the vector even if f always returns false. To prevent that, you can first iterate over the vector yourself and then only call retain if your condition is false for some element.

To:
Note that this clones the vector even if f always returns true. To prevent that, you can first iterate over the vector yourself and then only call retain if your condition is false for some element.

This makes more sense, since if f always returns true, nothing is changed, so common sense would be that the vector wouldn't be cloned, but this comment says otherwise.