Add a remark to AddEvent method (#4871)

* Add a remark to AddEvent method

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

Co-authored-by: Genevieve Warren <[email protected]>

Author: tarekgh

xml/System.Diagnostics/Activity.xml

@@ -172,7 +172,17 @@ This key/value pair is included in the collection returned by the <xref:System.D
         <summary>Adds the specified activity event to the events list.</summary>
         <returns>
           <see langword="this" /> for convenient chaining.</returns>
-        <remarks>To be added.</remarks>
+        <remarks>
+          <format type="text/markdown"><![CDATA[
+
+## Remarks
+
+`AddEvent` is not intended for adding many events over a long period of time, because the event objects are stored in memory. Adding too many events to the same `Activity` object can impact app performance.
+Callers should either restrict the number of events added to the same `Activity` object or choose another logging mechanism, for example, <xref:Microsoft.Extensions.Logging?displayProperty=fullName>.
+
+          ]]></format>
+      </remarks>
+
       </Docs>
     </Member>
     <Member MemberName="AddTag">