SetParent doesn't work after CreateActivity (dotnet#8048)

Author: noahfalk

xml/System.Diagnostics/Activity.xml

@@ -1202,6 +1202,7 @@ The specified format will take precedence over `Parent.IdFormat`, the <xref:Syst
 ## Remarks
 
 - This method should only be used in 'boundary' scenarios where an <xref:System.Diagnostics.Activity> from another process logically started this <xref:System.Diagnostics.Activity>. The `parentId` brings up the <xref:System.Diagnostics.Activity.Tags> (as well as the <xref:System.Diagnostics.Activity.ParentId> property) and can be used to reconstruct the causal tree.
+- This method is only intended for use with Activities created from the Activity constructor. Activities created by calling <xref:System.Diagnostics.ActivitySource.CreateActivity%2A> or <xref:System.Diagnostics.ActivitySource.StartActivity%2A> already have the parent ID set, and invoking this method has no effect.
 - This method should only be used before starting the <xref:System.Diagnostics.Activity> object. This method has no effect if you call it after the <xref:System.Diagnostics.Activity> object has started.
 
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