Document default unit of DateTime.utc_now/1 (#13532)

Author: sabiwara

lib/elixir/lib/calendar/datetime.ex

@@ -170,6 +170,9 @@ defmodule DateTime do
   truncate the resulting datetime. This is available
   since v1.15.0.
 
+  The default unit if none gets passed is `:native`,
+  which results on a default resolution of microseconds.
+
   ## Examples
 
       iex> datetime = DateTime.utc_now()