In case function has no docs, use the ones from callback

lib/ex_doc/retriever.ex

@@ -236,14 +236,7 @@ defmodule ExDoc.Retriever do
     function = actual_def(name, arity, type)
     line = find_actual_line(module_info.abst_code, function, :function) || doc_line
 
-    behaviour = Map.get(module_info.impls, {name, arity})
-
-    doc =
-      if is_nil(doc) && behaviour do
-        "Callback implementation for `c:#{inspect behaviour}.#{name}/#{arity}`."
-      else
-        doc
-      end
+    doc = docstring(doc, name, arity, Map.fetch(module_info.impls, {name, arity}))
 
     specs = module_info.specs
             |> Map.get(function, [])
@@ -272,6 +265,25 @@ defmodule ExDoc.Retriever do
     end
   end
 
+  defp docstring(nil, name, arity, {:ok, behaviour}) do
+    callback_docs = Code.get_docs(behaviour, :callback_docs) || []
+    case List.keyfind(callback_docs, {name, arity}, 0) do
+      {{^name, ^arity}, _, :callback, callback_docs} when is_binary(callback_docs) ->
+        [header, body] = String.split(callback_docs, "\n", parts: 2)
+        info = "Documentation for callback `c:#{inspect behaviour}.#{name}/#{arity}`."
+        "#{header}\n\n#{info}\n\n#{trim_leading_newlines(body)}"
+      _ ->
+        "Callback implementation for `c:#{inspect behaviour}.#{name}/#{arity}`."
+    end
+  end
+
+  defp docstring(doc, _name, _arity, _behaviour) do
+    doc
+  end
+
+  defp trim_leading_newlines("\n" <> rest), do: trim_leading_newlines(rest)
+  defp trim_leading_newlines(rest), do: rest
+
   defp get_callbacks(%{type: :behaviour, name: name, abst_code: abst_code}, source) do
     optional_callbacks = get_optional_callbacks(abst_code)
     (Code.get_docs(name, :callback_docs) || [])

test/ex_doc/retriever_test.exs

@@ -196,6 +196,10 @@ defmodule ExDoc.RetrieverTest do
       "A doc for this so it doesn't use 'Callback implementation for'"
     assert Enum.at(docs, 1).doc ==
       "Callback implementation for `c:CustomBehaviourOne.greet/1`."
+    assert Enum.at(docs, 2).doc ==
+      "This is a sample callback.\n\n" <>
+      "Documentation for callback `c:CustomBehaviourOne.hello/1`.\n\n" <>
+      "With description\n"
   end
 
   ## PROTOCOLS

test/fixtures/behaviour.ex

@@ -4,6 +4,8 @@ defmodule CustomBehaviourOne do
 
   @doc """
   This is a sample callback.
+
+  With description
   """
   @callback hello(integer) :: integer
   @callback greet(integer | String.t) :: integer