Add missing doc since (#12623)

Courtesy of https://github.com/wojtekmach/app_manifest. :)

Author: wojtekmach

lib/elixir/lib/code.ex

@@ -562,6 +562,7 @@ defmodule Code do
       Defaults to `false`.
 
   """
+  @doc since: "1.15.0"
   @spec with_diagnostics(keyword(), (-> result)) :: {result, [diagnostic(:warning | :error)]}
         when result: term()
   def with_diagnostics(opts \\ [], fun) do

lib/mix/lib/mix.ex

@@ -589,6 +589,7 @@ defmodule Mix do
   list the Erlang application dependencies under the `:extra_applications`
   section of your `mix.exs`.
   """
+  @doc since: "1.15.0"
   def ensure_application!(app) when is_atom(app) do
     ensure_application!(app, Mix.State.builtin_apps())
     :ok