Add explicit example to allow nesting lists within lists (#755)

Author: spawnia

spec/Section 3 -- Type System.md

@@ -1554,8 +1554,10 @@ Input object type extensions have the potential to be invalid if incorrectly def
 A GraphQL list is a special collection type which declares the type of each
 item in the List (referred to as the *item type* of the list). List values are
 serialized as ordered lists, where each item in the list is serialized as per
-the item type. To denote that a field uses a List type the item type is wrapped
-in square brackets like this: `pets: [Pet]`.
+the item type.
+
+To denote that a field uses a List type the item type is wrapped in square brackets
+like this: `pets: [Pet]`. Nesting lists is allowed: `matrix: [[Int]]`.
 
 **Result Coercion**