Update System.Exception.StackTrace remark to reflect the new behavior for rethrown exceptions

[ateoi]

The line below needs to be updated to reflect the fix for dotnet/coreclr#15780 and implemented by dotnet/coreclr#16464. This change is expected to be included in .NET Core 2.1 - https://github.com/dotnet/coreclr/issues/15780#issuecomment-368585175.

This is the new behavior for rethrown exceptions:

1. Exception rethrown in a method different than the method where it was originally thrown: the stack trace contains both the location in the method where the exception was originally thrown, and the location where the method that threw the exception was called.

2. Exception is thrown and later rethrown in the same method: the stack trace only contains the location where the exception was originally thrown and does not include the location where the exception was rethrown.

https://github.com/dotnet/docs/blob/f66fe50bf0c637bfd9ca305c7ca30c3f37e50620/xml/System/Exception.xml#L1061

[BillWagner]

Note: the link in the issue description points to the location when System.XML was in the dotnet/docs repo.