Improve warning as errors deprecation notice

Author: josevalim

lib/elixir/lib/code.ex

@@ -1709,7 +1709,9 @@ defmodule Code do
   def put_compiler_option(:warnings_as_errors, _value) do
     IO.warn(
       ":warnings_as_errors is deprecated as part of Code.put_compiler_option/2, " <>
-        "pass it as option to Kernel.ParallelCompiler instead"
+        "pass it as option to Kernel.ParallelCompiler instead or as a --warnings-as-errors flag. " <>
+        "If you need to set it as a default in a mix task, you can also set it under aliases: " <>
+        "[compile: \"compile --warnings-as-errors\"]"
     )
 
     :ok