update foreach for ref variables. (dotnet#6202)

* update foreach for `ref` variables.

* respond to feedback.

docs/csharp/language-reference/keywords/foreach-in.md

@@ -1,6 +1,6 @@
 ---
 title: "foreach, in (C# Reference)"
-ms.date: 05/24/2018
+ms.date: 06/28/2018
 f1_keywords: 
   - "foreach"
   - "foreach_CSharpKeyword"
@@ -31,6 +31,10 @@ The next example uses the `foreach` statement with an instance of the <xref:Syst
 
 [!code-csharp-interactive[span example](~/samples/snippets/csharp/keywords/IterationKeywordsExamples.cs#2)]
 
+Beginning with C# 7.3, when the collection type supports `ref` access to its elements, you can declare the iteration variable with the `ref` or `ref readonly` modifier. The following example uses a `ref` iteration variable to set the value of each item in a stackalloc array. The `ref readonly` version iterates the collection to print all the values. The `readonly` declaration uses an implicit local variable declaration. Implicit variable declarations can be used with either `ref` or `ref readonly` declarations, as can explicitly typed variable declarations.
+
+[!code-csharp-interactive[ref span example](~/samples/snippets/csharp/keywords/IterationKeywordsExamples.cs#RefSpan)]
+
 ## C# language specification
 
 [!INCLUDE[CSharplangspec](~/includes/csharplangspec-md.md)]

docs/csharp/whats-new/csharp-7-3.md

@@ -87,7 +87,7 @@ ref VeryLargeStruct refLocal = ref veryLargeStruct; // initialization
 refLocal = ref anotherVeryLargeStruct; // reassigned, refLocal refers to different storage.
 ```
 
-For more information, see the article on [`ref` returns and `ref` locals](../programming-guide/classes-and-structs/ref-returns.md).
+For more information, see the article on [`ref` returns and `ref` locals](../programming-guide/classes-and-structs/ref-returns.md), and the article on [`foreach`](../language-reference/keywords/foreach-in.md).
 
 ### `stackalloc` arrays support initializers