Add examples of a spread element to the "Collection initializers" article (#44045)

* Add example of spread operator for collection initializers

* Inlcude the spread element example in the text

* Include additional syntax for manually creating additional element

* Include spread element in full example code

* Remove unnecessary System.Xml.Linq

Author: BartoszKlonowski

docs/csharp/programming-guide/classes-and-structs/object-and-collection-initializers.md

@@ -149,6 +149,14 @@ You can specify [null](../../language-reference/keywords/null.md) as an element
 
 :::code language="csharp" source="./snippets/object-collection-initializers/BasicObjectInitializers.cs" id="ListInitializerWithNull":::
 
+You can use a spread element to create one list that copies other list or lists.
+
+:::code language="csharp" source="./snippets/object-collection-initializers/BasicObjectInitializers.cs" id="ListInitializerWithSpreadOperator":::
+
+And include additonal elements along with using a spread element.
+
+:::code language="csharp" source="./snippets/object-collection-initializers/BasicObjectInitializers.cs" id="ListInitializerWithSpreadOperatorAndAdditionalElement":::
+
 You can specify indexed elements if the collection supports read / write indexing.
 
 :::code language="csharp" source="./snippets/object-collection-initializers/BasicObjectInitializers.cs" id="DictionaryIndexerInitializer":::

docs/csharp/programming-guide/classes-and-structs/snippets/object-collection-initializers/BasicObjectInitializers.cs

@@ -98,6 +98,14 @@ from p in products
         };
         // </SnippetListInitializerWithNull>
 
+        // <SnippetListInitializerWithSpreadOperator>
+        List<Cat> allCats = [.. cats, .. moreCats];
+        // </SnippetListInitializerWithSpreadOperator>
+
+        // <SnippetListInitializerWithSpreadOperatorAndAdditionalElement>
+        List<Cat> additionalCats = [.. cats, new Cat { Name = "Furrytail", Age = 5 }, .. moreCats];
+        // </SnippetListInitializerWithSpreadOperatorAndAdditionalElement>
+
         // <SnippetDictionaryIndexerInitializer>
         var numbers = new Dictionary<int, string>
         {
@@ -182,15 +190,10 @@ public static void Main()
             null
         };
 
-        // Display results.
-        System.Console.WriteLine(cat.Name);
+        List<Cat> allCats = [.. cats, new Cat { Name = "Łapka", Age = 5 }, cat, .. moreCats];
 
-        foreach (Cat c in cats)
-        {
-            System.Console.WriteLine(c.Name);
-        }
-
-        foreach (Cat? c in moreCats)
+        // Display results.
+        foreach (Cat? c in allCats)
         {
             if (c != null)
             {
@@ -203,13 +206,14 @@ public static void Main()
         }
     }
     // Output:
-    //Fluffy
-    //Sylvester
-    //Whiskers
-    //Sasha
-    //Furrytail
-    //Peaches
-    //List element has null value.
+    // Sylvester
+    // Whiskers
+    // Sasha
+    // Łapka
+    // Fluffy
+    // Furrytail
+    // Peaches
+    // List element has null value.
 }
 // </SnippetFullExample>