Explicitly mention arrays are ordered

Kind-of obvious, but doesn't hurt to explicitly mention it, as it adds
just one word. Rephrased slightly for clarity.

Fixes toml-lang#1027

toml.md

@@ -641,9 +641,9 @@ rounded.
 
 ## Array
 
-Arrays are square brackets with values inside. Whitespace is ignored. Elements
-are separated by commas. Arrays can contain values of the same data types as
-allowed in key/value pairs. Values of different types may be mixed.
+Arrays are ordered values surrounded by square brackets. Whitespace is ignored.
+Elements are separated by commas. Arrays can contain values of the same data
+types as allowed in key/value pairs. Values of different types may be mixed.
 
 ```toml
 integers = [ 1, 2, 3 ]