Update audiences/index.md

Add specificity that time comparison operators apply only to custom traits

Author: samkcrespo

src/engage/audiences/index.md

@@ -60,7 +60,7 @@ You can use the following time comparison operators in your audience definition:
 - `before last`
 - `after next` 
 
-Only ISO timestamps can be used with these operators.
+Only ISO timestamps can be used with these operators. Additionally, these time comparison operators exclusively apply to custom traits.
 If the timestamp is not a valid ISO timestamp (for example, a trailing `Z` is missing), Segment won't process the audience in real-time. Learn more about [real-time compute compared to batch](docs/engage/audiences/#real-time-compute-compared-to-batch)
 
 ### Funnel Audiences