Return the metadata of elements to complete at the end of STRING.
This metadata is an alist. Currently understood keys are:
- `category': the kind of objects returned by `all-completions'.
Used by `completion-category-overrides'.
- `annotation-function': function to add annotations in *Completions*.
Takes one argument (STRING), which is a possible completion and
returns a string to append to STRING.
- `display-sort-function': function to sort entries in *Completions*.
Takes one argument (COMPLETIONS) and should return a new list
of completions. Can operate destructively.
- `cycle-sort-function': function to sort entries when cycling.
Works like `display-sort-function'.
The metadata of a completion table should be constant between two boundaries. (fn STRING TABLE PRED)