Improve docs for chunk_by/4

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

lib/elixir/lib/enum.ex

@@ -374,8 +374,15 @@ defmodule Enum do
   end
 
   @doc """
-  Splits enumerable on every element for which `fun` returns a new
-  value.
+  Chunks the `enum` with fine grained control when every chunk is emitted.
+
+  `chunk_fun` receives the current element and the accumulator and
+  must return `{:cont, element, acc}` to emit the given chunk and
+  continue with accumulator or `{:cont, acc}` to not emit any chunk
+  and continue with the return accumulator.
+
+  `after_fun` is invoked when iteration is done and must also return
+  `{:cont, element, acc}` or `{:cont, acc}`.
 
   Returns a list of lists.