DOC: statistics/2: order keys alphabetically and add `predicates` to the docs.
DOC: statistics/2: order keys alphabetically and add `predicates` to the
docs.

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