tweak activation wording to focus around policy not groups

Signed-off-by: R.I.Pienaar <[email protected]>

adr/ADR-42.md

@@ -46,8 +46,8 @@ We introduce 2 settings on `ConsumerConfig` that activates these related feature
 }
 ```
 
-The presence of the `PriorityGroups` setting activates the set of features documented here with a list of names supported
-behaviours configured using the `PriorityPolicy`.
+The presence of the `PriorityPolicy` set to a known policy activates the set of features documented here, `PriorityGroups`
+require at least one entry.
 
 Technically in some of the proposed policies the `PriorityGroups` have no real meaning today, but we keep it for consistency 
 and allow for future features to be added that would be per-group without then requiring all clients to be updated.