Update ActivityUserGuide.md (#49818)

Refer activity guide to the official docs dotnet/docs#23313 is bringing more enhancements shortly
dotnet · Mar 19, 2021 · 9f93bcb · 9f93bcb
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 # Activity User Guide
 
+**This doc is being obsoleted by: https://docs.microsoft.com/dotnet/core/diagnostics/distributed-tracing
+Future doc changes should be done in the official docs, not here.
+There is still some information here that is not present in the official docs (yet) so I am preserving it as-is.**
+
 This document describes Activity, a class that allows storing and accessing diagnostics context and consuming it with logging system.
 
 This document provides Activity architecture [overview](#overview) and [usage](#activity-usage).