If I'm not mistaken GIN will replace posting list to posting tree (and so shift it to another page) when on a current page there is not enough space. But this does not necessarily mean that page is not enough to accommodate current tuple(1). This rather means space left on a page is not necessary due to occupation by neighbouring tuples. (2)
If we just replace tuple -> page we'll get meaning (1) which is not precisely so.
I think existing description is accurate and doesn't need change basically.