Increase understandability of the tuple documentation (#9226)

[ci skip]

Since Elixir data structures are immutable, emphasize that creating a new tuple
is required. Also add a "result" binding to make it more clear that the last
line is being used as a result for later.

Author: axelson

lib/elixir/lib/tuple.ex

@@ -31,16 +31,17 @@ defmodule Tuple do
 
   The functions in this module that add and remove elements from tuples are
   rarely used in practice, as they typically imply tuples are being used as
-  collections. To append to a tuple, it is preferable to use pattern matching:
+  collections. To append to a tuple, it is preferable to extract the elements
+  from the old tuple with pattern matching, and then create a new tuple:
 
       tuple = {:ok, :example}
 
       # Avoid
-      Tuple.insert_at(tuple, 2, %{})
+      result = Tuple.insert_at(tuple, 2, %{})
 
       # Prefer
       {:ok, atom} = tuple
-      {:ok, atom, %{}}
+      result = {:ok, atom, %{}}
 
   """