enhance documentation on when the TestCleanup is executed (#709)

To me it was unclear if it's once per `[TestClass]` or once per `[TestMethod]`.

src/TestFramework/MSTest.Core/Attributes/VSTestAttributes.cs

@@ -120,7 +120,7 @@ public sealed class TestInitializeAttribute : Attribute
     }
 
     /// <summary>
-    /// The test cleanup attribute.
+    /// The test cleanup attribute marks methods that are executed after every test marked with a <see cref="TestMethodAttribute"/>.
     /// </summary>
     [AttributeUsage(AttributeTargets.Method, AllowMultiple = false)]
     public sealed class TestCleanupAttribute : Attribute