Wrapper hook around `completion-in-region'.
The functions on this special hook are called with 5 arguments:
NEXT-FUN START END COLLECTION PREDICATE.
NEXT-FUN is a function of four arguments (START END COLLECTION PREDICATE)
that performs the default operation. The other four arguments are like
the ones passed to `completion-in-region'. The functions on this hook
are expected to perform completion on START..END using COLLECTION
and PREDICATE, either by calling NEXT-FUN or by doing it themselves.