Remark on obsolete APIs (#4980)

Author: gewarren

xml/System/MarshalByRefObject.xml

@@ -214,12 +214,16 @@
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
-## Remarks  
- For more information about lifetime services, see the <xref:System.Runtime.Remoting.Lifetime.LifetimeServices> class.  
+## Remarks
+
+This method is marked obsolete starting in .NET 5.0.
+
+For more information about lifetime services, see the <xref:System.Runtime.Remoting.Lifetime.LifetimeServices> class.  
   
  ]]></format>
         </remarks>
         <exception cref="T:System.Security.SecurityException">The immediate caller does not have infrastructure permission.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+ only: In all cases.</exception>
       </Docs>
     </Member>
     <Member MemberName="InitializeLifetimeService">
@@ -265,11 +269,12 @@
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
-## Remarks  
- For more information about lifetime services, see the <xref:System.Runtime.Remoting.Lifetime.LifetimeServices> class.  
-  
-   
-  
+## Remarks
+
+This method is marked obsolete starting in .NET 5.0.
+
+For more information about lifetime services, see the <xref:System.Runtime.Remoting.Lifetime.LifetimeServices> class.
+
 ## Examples  
  The following code example demonstrates creating a lease.  
   
@@ -280,6 +285,7 @@
  ]]></format>
         </remarks>
         <exception cref="T:System.Security.SecurityException">The immediate caller does not have infrastructure permission.</exception>
+        <exception cref="T:System.PlatformNotSupportedException">.NET Core and .NET 5+ only: In all cases.</exception>
       </Docs>
     </Member>
     <MemberGroup MemberName="MemberwiseClone">