Add missing exception to Uri.IdnHost (#2658)

Uri.IdnHost throws InvalidOperationException if the URI is not absolute. This is because it internally forwards to `DnsSafeHost` which throws. This is true for .NET Core 3+ (at least) and .NET Framework 4.8 (Reference Source).

Sources:
https://github.com/dotnet/corefx/blob/master/src/System.Private.Uri/src/System/Uri.cs#L1219-L1233
https://referencesource.microsoft.com/#System/net/System/URI.cs,988f6bd791dbb0dc,references

Author: watfordgnf

xml/System/Uri.xml

@@ -2457,6 +2457,7 @@ If you used an escaped string to construct this instance (for example, `"http://
 
  ]]></format>
         </remarks>
+        <exception cref="T:System.InvalidOperationException">This instance represents a relative URI, and this property is valid only for absolute URIs.</exception>
       </Docs>
     </Member>
     <Member MemberName="IsAbsoluteUri">
@@ -4901,4 +4902,4 @@ If you used an escaped string to construct this instance (for example, `"http://
       </Docs>
     </Member>
   </Members>
-</Type>
+</Type>