Add a couple of missing items to the release notes (#1938)

Author: idg10

Rx.NET/Documentation/ReleaseHistory/Rx.v6.md

@@ -11,4 +11,6 @@
 
 * Tested against .NET 6, and .NET 7
 * When unhandled exceptions from `Task` used to cause `TaskScheduler.UnobservedExceptions`, applications can opt into swallowing failures silently (to be consistent with how Rx has always handled unhandled exceptions in the equivalent non-Task-oriented scenarios; this only applies to cases in which Rx has no way of reporting the exception, typically because the relevant observable no longer has any subscribers on which we could call `OnError`)
-* `SingleAssignmentDisposableValue` type is now public
+* `SingleAssignmentDisposableValue` type is now public
+* Trimming annotations now present
+* debug symbols now available as separate `snupkg` instead of being built in, reducing assembly size