Enhance docs and test

Author: reakaleek

docs/source/syntax/tabs.md

@@ -97,6 +97,8 @@ Content for C# tab
 
 #### Result
 
+##### Grouped Tabs
+
 ::::{tab-set}
 :group: languages
 :::{tab-item} Java
@@ -134,6 +136,8 @@ Other content for C# tab
 :::
 ::::
 
+##### Ungrouped Tabs
+
 ::::{tab-set}
 :::{tab-item} Java
 :sync: java

tests/Elastic.Markdown.Tests/Directives/TabTests.cs

@@ -164,4 +164,15 @@ public void ParsesGroup()
 			t.GetGroupKey().Should().Be("languages");
 		}
 	}
+
+	[Fact]
+	public void ParsesSyncKey()
+	{
+		var set = Document.OfType<TabSetBlock>().First();
+		var items = set.OfType<TabItemBlock>().ToArray();
+		items.Should().HaveCount(3);
+		items[0].SyncKey.Should().Be("java");
+		items[1].SyncKey.Should().Be("golang");
+		items[2].SyncKey.Should().Be("csharp");
+	}
 }