Clarify behaviour for AggregateExceptions in the SetBeforeSend hook (#14265)

Author: jamescrosswell

docs/platforms/dotnet/common/configuration/options.mdx

@@ -225,6 +225,8 @@ This function is called with an SDK-specific message or error event object, and
 
 By the time <PlatformIdentifier name="set-before-send" /> is executed, all scope data has already been applied to the event. Further modification of the scope won't have any effect.
 
+When filtering based on exception types, the original exception (including any `AggregateException`) will be stored in `SentryEvent.Exception`. However, Sentry's `MainExceptionProcessor` unpacks AggregateExceptions automatically, and so what you will see in Sentry will be the various inner exceptions. These are also available in the SDK by inspecting the `SentryEvent.SentryExceptions` collection.
+
 </ConfigKey>
 
 <ConfigKey name="set-before-breadcrumb">