dotnet: explain why transactions are never global

Co-authored-by: James Crosswell <[email protected]>

Author: mattico

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

@@ -55,7 +55,7 @@ Broadly, the ExecutionContext passes to child tasks and threads, but not to pare
 
 When Global Mode is **enabled**, a single scope stack is shared by the whole application.
 
-Since version 5.0.0 the transaction is always set on the current ExecutionContext, regardless of the Global Mode.
+Since version 5.0.0 the transaction is always set on the current ExecutionContext, regardless of the Global Mode, so that spans from the UI don't get mixed up with transactions in background services.
 
 See the <PlatformLink to="/enriching-events/scopes/">Scopes and Hubs documentation</PlatformLink> for more information.