docs: mstest support for IsNull, IsNotNull, IsInstanceOfType, IsNotInstanceOfType

Author: Meir017

docs/MsTestAnalyzer.md

@@ -80,13 +80,13 @@ obj.Should().BeNull();
 #### Failure messages
 
 ```cs
-var obj = new object();
+var obj = "test";
 
 // old assertion:
 Assert.IsNull(obj); 	// fail message: Assert.IsNull failed. 
 
 // new assertion:
-obj.Should().BeNull(); 	// fail message: Expected obj to be <null>, but found System.Object (HashCode=53299218).
+obj.Should().BeNull(); 	// fail message: Expected obj to be <null>, but found "test".
 ```
 
 ### scenario: ObjectAssertIsNotNull

src/FluentAssertions.Analyzers.FluentAssertionAnalyzerDocs/MsTestAnalyzerTests.cs

@@ -93,7 +93,7 @@ public void ObjectAssertIsNull()
     public void ObjectAssertIsNull_Failure_OldAssertion()
     {
         // arrange
-        var obj = new object();
+        var obj = "test";
 
         // old assertion:
         Assert.IsNull(obj);
@@ -103,7 +103,7 @@ public void ObjectAssertIsNull_Failure_OldAssertion()
     public void ObjectAssertIsNull_Failure_NewAssertion()
     {
         // arrange
-        var obj = new object();
+        var obj = "test";
 
         // new assertion:
         obj.Should().BeNull();