Small Design Guideline Update: Overriding Exception.ToString

One of the folks on my team just added a minor update to
the Design Guidelines document about overloading ToString() on Exceptions. 
As always, comments are welcome.

           
Do override ToString when your
exception provides extra properties."urn:schemas-microsoft-com:office:office" />

The code handling the custom exception may not have
caught your specific type of exception, and therefore does not have access to
the extra information unless you provide it in
ToString.