DOC: Document the behaviour of group_pairs_by_key/4
DOC: Document the behaviour of group_pairs_by_key/4

Originally, the documentation demanded that the first argument is
a list of pairs sorted to the standard order of terms on the keys,
an unnecessary limitation. Added an example of using the predicate
together with the new sorting predicate sort/4.

edechter/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,edechter/swipl-devel,edechter/swipl-devel