null cannot be boxed (dotnet#5335)

The use of the term boxing is not appropriate in case of 'null'.

Author: sgurukrupa

xml/System/Nullable.xml

@@ -54,7 +54,9 @@
  The <xref:System.Nullable> class provides complementary support for the <xref:System.Nullable%601> structure. The <xref:System.Nullable> class supports obtaining the underlying type of a nullable type, and comparison and equality operations on pairs of nullable types whose underlying value type does not support generic comparison and equality operations.  
   
 ## Boxing and Unboxing  
- When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the <xref:System.Nullable%601> object, not the <xref:System.Nullable%601> object itself. That is, if the <xref:System.Nullable%601.HasValue%2A> property is `true`, the contents of the <xref:System.Nullable%601.Value%2A> property is boxed. If the `HasValue` property is `false`, `null` is boxed. When the underlying value of a nullable type is unboxed, the common language runtime creates a new <xref:System.Nullable%601> structure initialized to the underlying value.  
+ When a nullable type is boxed, the common language runtime automatically boxes the underlying value of the <xref:System.Nullable%601> object, not the <xref:System.Nullable%601> object itself. That is, if the <xref:System.Nullable%601.HasValue%2A> property is `true`, the contents of the <xref:System.Nullable%601.Value%2A> property is boxed.
+ 
+ If the `HasValue` property of a nullable type is `false`, the result of the boxing operation is `null`. When the underlying value of a nullable type is unboxed, the common language runtime creates a new <xref:System.Nullable%601> structure initialized to the underlying value.  
   
  ]]></format>
     </remarks>