Elaborate on example for report_dropped rejection case

The protocol requires Aggregators to prevent report replays within a
task. It also allows the Aggregators to eventually clean up state for
anti-replay. If a report arrives late, it may be rejected because the
Aggregator can't determine if it has been replayed. Clarify this in the
example.

draft-ietf-ppm-dap.md

@@ -1915,9 +1915,9 @@ following checks:
 
 1. Finally, if an Aggregator cannot determine if an input share is valid, it
    MUST mark the input share as invalid with error `report_dropped`. For
-   example, if the Aggregator has evicted the state required to perform the
-   check from long-term storage. (See {{reducing-storage-requirements}} for
-   details.)
+   example, the report timestamp may be so far in the past that the Aggregator
+   has evicted the state required to perform the check from long-term storage.
+   (See {{reducing-storage-requirements}} for details.)
 
 If all of the above checks succeed, the input share is not marked as invalid.