Miscellaneous fixes in IntelliSense comments (#301)

***NO_CI***

Author: josesimoes

source/TestFramework/Assert.cs

@@ -229,6 +229,11 @@ public static void IsTrue(bool condition, [CallerArgumentExpression(nameof(condi
             }
         }
 
+        /// <summary>
+        /// Call this method to indicate that a test should be skipped.
+        /// </summary>
+        /// <param name="message">The message to include in the exception when the test is skipped. The message is shown in test results.</param>
+        /// <exception cref="SkipTestException">Thrown to indicate that a test should be skipped.</exception>
         [DoesNotReturn]
         public static void SkipTest(string message = null)
         {

source/TestFramework/CollectionAssert.cs

@@ -2,6 +2,7 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 
 
+using System;
 using System.Collections;
 using TestFrameworkShared;
 
@@ -28,7 +29,7 @@ public sealed class CollectionAssert
         /// <param name="collection">The collection the test expects to be empty.</param>
         /// <param name="message">The message to include in the exception when the collection is empty. The message is shown in test results.</param>
         /// <exception cref="AssertFailedException">Raises an exception if the collection is not empty.</exception>
-        /// <exception cref=""></exception>
+        /// <exception cref="ArgumentNullException">Raises an exception if the collection is null.</exception>
         public static void Empty(ICollection collection, string message = "")
         {
             Assert.EnsureParameterIsNotNull(collection, "CollectionAssert.Empty");