Add missing @doc since (#11319)

Author: wojtekmach

lib/elixir/lib/kernel.ex

@@ -4125,6 +4125,7 @@ defmodule Kernel do
       4.0
 
   """
+  @doc since: "1.13.0"
   @spec integer ** non_neg_integer :: integer
   @spec integer ** neg_integer :: float
   @spec float ** float :: float

lib/elixir/lib/macro.ex

@@ -728,6 +728,7 @@ defmodule Macro do
       [{:foo, [], [1, "abc"]}, 1, "abc"]
 
   """
+  @doc since: "1.13.0"
   def prewalker(ast) do
     &prewalker([ast], &1, &2)
   end
@@ -784,6 +785,7 @@ defmodule Macro do
       [1, "abc", {:foo, [], [1, "abc"]}]
 
   """
+  @doc since: "1.13.0"
   def postwalker(ast) do
     &postwalker([ast], make_ref(), &1, &2)
   end