- add `{:halt, acc}` tuple return doc for `chunk_fun`
- explain `after_fun` semantics in more detail
- format return tuples for `chunk_fun` and `after_fun` as markdown lists
- added note that `acc` in `after_fun` return tuple is ignored
- note that `Enum.chunk_while/4` returns a list of *chunks*, not a list
of lists (emitted chunks can be anything, not necessarily just lists)