Update UnmanagedType.xml (dotnet#8837)

Author: DJm00n

xml/System.Runtime.InteropServices/UnmanagedType.xml

@@ -619,7 +619,7 @@
       </ReturnValue>
       <MemberValue>47</MemberValue>
       <Docs>
-        <summary>A Windows Runtime string. You can use this member on the <see cref="T:System.String" /> data type.</summary>
+        <summary>A Windows Runtime string. You can use this member on the <see cref="T:System.String" /> data type. [Built-in support for WinRT was removed in .NET 5](https://learn.microsoft.com/dotnet/core/compatibility/interop/5.0/built-in-support-for-winrt-removed). See [Formerly built-in supported types](https://learn.microsoft.com/dotnet/standard/native-interop/best-practices#formerly-built-in-supported-types) for workaround.</summary>
       </Docs>
     </Member>
     <Member MemberName="I1">
@@ -909,7 +909,7 @@
       </ReturnValue>
       <MemberValue>46</MemberValue>
       <Docs>
-        <summary>A Windows Runtime interface pointer. You can use this member on the <see cref="T:System.Object" /> data type.</summary>
+        <summary>A Windows Runtime interface pointer. You can use this member on the <see cref="T:System.Object" /> data type. [Built-in support for WinRT was removed in .NET 5](https://learn.microsoft.com/dotnet/core/compatibility/interop/5.0/built-in-support-for-winrt-removed).</summary>
       </Docs>
     </Member>
     <Member MemberName="Interface">