Add default formats (#4502)

Author: gewarren

xml/System.Diagnostics/Activity.xml

@@ -326,6 +326,8 @@ This key/value pair is included in the collection returned by the <xref:System.D
 
 The `Activity` instance tries to use the same format for IDs as its parent. If the activity has no parent, this property determines the default format to use.
 
+In .NET 5.0 and later, the default format is <xref:System.Diagnostics.ActivityIdFormat.W3C?displayProperty=nameWithType>. In previous versions, the default format is <xref:System.Diagnostics.ActivityIdFormat.Hierarchical?displayProperty=nameWithType>.
+
          ]]></format>
         </remarks>
       </Docs>