As I understand it, the only time you would find -s' is in a plural noun that already ends in -s, as in:
'The Thompsons' dog is playing outside.' (the dog belong to the Thompson family)
'The horses' saddles are in the barn.' (the saddles belonging to all of the horses)
So "Chaucers' poems" is saying there are several Chaucers. '... Small section of Geoffrey Chaucer's' is the small section of works by the one Geoffrey Chaucer. I think this is made even clearer if you imagined another word at the end, like in 'a small section of Geoffrey Chaucer's works.'
Actually I just now read that you also find -s' in singular personal names that end in -s, where you don't pronounce an extra 's' when saying the possessive in spoken language, like in
'Jeff Bridges' latest film'
(not a teacher, just a language lover)
Student or Learner