Include definition of tuples for destructuring.

Updates the tutorial to include a simple definition for tuples in
section 4.2. Fixes #5132.

Author: Bryan Dunsmore

doc/tutorial.md

@@ -495,7 +495,10 @@ omitted.
 
 A powerful application of pattern matching is *destructuring*:
 matching in order to bind names to the contents of data
-types. Assuming that `(float, float)` is a tuple of two floats:
+types.
+
+> ***Note:*** The following code makes use of tuples (`(float, float)`) which
+> are explained later. For now you can think of tuples as a list of items.
 
 ~~~~
 fn angle(vector: (float, float)) -> float {