fix callback summary being output in reverse order

lib/ex_doc/retriever.ex

@@ -88,8 +88,9 @@ defmodule ExDoc.Retriever do
     if type == :behaviour do
       callbacks = Kernel.Typespec.beam_callbacks(module)
 
-      docs = Enum.map(module.__behaviour__(:docs),
-        get_callback(&1, source_path, source_url, callbacks)) ++ docs
+      callback_docs = Enum.map(module.__behaviour__(:docs),
+        get_callback(&1, source_path, source_url, callbacks))
+      docs = docs ++ Enum.reverse(callback_docs)
     end
 
     ExDoc.ModuleNode[

test/exdoc/retriever_test.exs

@@ -117,7 +117,7 @@ defmodule ExDoc.RetrieverTest do
   test "ignore behaviours internal functions" do
     [node] = get_docs :modules, ["CustomBehaviour"]
     functions = Enum.map node.docs, fn(doc) -> doc.id end
-    assert functions == ["hello/1"]
+    assert functions == ["first/1", "hello/1","last/1"]
     assert hd(node.docs).type == :defcallback
     assert hd(node.docs).signature == [{ :integer, [line: 7], [] }]
   end

test/fixtures/behaviour.ex

@@ -1,8 +1,18 @@
 defmodule CustomBehaviour do
   use Behaviour
 
+  @doc """
+  I should be first.
+  """
+  defcallback first(integer) :: integer
+
   @doc """
   This is a sample callback.
   """
   defcallback hello(integer) :: integer
-end
+
+  @doc """
+  I should be last.
+  """
+  defcallback last(integer) :: integer
+end