The recommendation seems to suggest that it could defer one or more *whole* footnotes, if there is not enough space in the page where their citations are, even if this is not very usual to happen in books;
Actually, this is *very* unusual, and can only happen if there is only a single line of content and all the footnote space is already eaten up, either by footnotes deferred from previous pages or because there are lots and/or very long footnotes in the same line.
The common convention is that only a footnote cited in the last content line of the bage may be broken across pages, and no further footnote content will be deferred to the next page. In fact, breaking footnotes should be avoided if the content line citating the footnote can be put onto the next page withouth leaving an ugly white area below the content, either by distributing surplus bpd space to before/after block spaces or by fiddling with line heigths (or both).
Duh, an example in the Wiki would be in order...