added clarification to Enum.uniq_by (#5483)

Author: venkatd

lib/elixir/lib/enum.ex

@@ -2555,6 +2555,8 @@ defmodule Enum do
   The function `fun` maps every element to a term which is used to
   determine if two elements are duplicates.
 
+  The first occurrence of each element is kept.
+
   ## Example
 
       iex> Enum.uniq_by([{1, :x}, {2, :y}, {1, :z}], fn {x, _} -> x end)