Introduce Report Data Lifetime (#1505)

* Revise to simple non-normative note

* reword, make into note

* adjusts note

Author: ThomasQuesadilla

index.bs

@@ -4662,6 +4662,14 @@ Issue: Specify this in terms of <a href="https://html.spec.whatwg.org/multipage/
 The user agent must periodically run [=queue reports for delivery=] on the
 [=event-level report cache=] and [=aggregatable attribution report cache=].
 
+The user agent may delete reports without sending them according
+to [=implementation-defined=] criteria.
+
+<p class=note>
+For example, this can be used to limit the maximum age of
+any report that is sent.
+</p>
+
 To <dfn>queue reports for delivery</dfn> given a [=set=] of
 [=attribution reports=] |cache|, run the following steps: