Fix Base.decode16!/2 doc: base 16 has no padding (#15558)

Author: nseaSeb

lib/elixir/lib/base.ex

@@ -536,7 +536,7 @@ defmodule Base do
     * `:lower` - only allows lower case characters
     * `:mixed` - allows mixed case characters
 
-  An `ArgumentError` exception is raised if the padding is incorrect or
+  An `ArgumentError` exception is raised if the string has an odd length or
   a non-alphabet character is present in the string.
 
   ## Examples