Index Initializer (#5577)

* Update csharp-6 

Modified Index Initializer verbiage to identify collection initializers were able to be used with Dictionaries previously. Tried to identify that the change makes use of the indexer versus calling the add method by remaining more consistent with index usage. Might want to update example or links to the Dictionary collection initializer page within docs.

* Index Initializer #5506 

Update typo and verbiage in accordance with reviewed comments.

Author: DevonAleshire

docs/csharp/whats-new/csharp-6.md

@@ -508,12 +508,12 @@ if any, is lost.
 ## Index Initializers
 
 *Index Initializers* is one of two features that make collection
-initializers more consistent. In earlier releases of C#, you could use
-*collection initializers* only with sequence style collections:
+initializers more consistent with index usage. In earlier releases of C#, you could use
+*collection initializers* only with sequence style collections, including <xref:System.Collections.Generic.Dictionary%602> by adding braces around key and value pairs:
 
 [!code-csharp[ListInitializer](../../../samples/snippets/csharp/new-in-6/initializers.cs#ListInitializer)]
 
-Now, you can also use them with <xref:System.Collections.Generic.Dictionary%602> collections and similar types:
+Now, you can use them with <xref:System.Collections.Generic.Dictionary%602> collections and similar types. The new syntax supports assignment using an index into the collection:
 
 [!code-csharp[DictionaryInitializer](../../../samples/snippets/csharp/new-in-6/initializers.cs#DictionaryInitializer)]