Document configuration changes that are allowed

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

adr/ADR-42.md

@@ -59,6 +59,11 @@ data.
 
 This is only supported on Pull Consumers, configuring this on a Push consumer must raise an error.
 
+Some aspects of configuration is updatable, called out in the respective sections, but we cannot support updating a 
+consumer from one with groups to one without and vice versa due to the internal state tracking that is required and 
+the fact that generally clients need adjustment to use these features. We have identified some mitigation approaches 
+to ease migration but will wait for user feedback. We also cannot switch between different policies.
+
 ## Priority Policies
 
 ### `overflow` policy
@@ -97,6 +102,8 @@ In the specific case where MaxAckPending is 1 and a pull is made using `min_pend
 there are no other pulls waiting. This means we have to give priority to pulls without conditions over those with when
 considering the next pull that will receive a message.
 
+Once multiple groups are supported consumer updates could add and remove groups.
+
 ### `pinned_client` policy
 
 Users want to have a single client perform all the work for a consumer, but they also want to have a stand-by client that
@@ -163,4 +170,7 @@ type PriorityGroupState struct {
 }
 ```
 
-Future iterations will include delivery stats per group.
+Future iterations will include delivery stats per group.
+
+Once multiple groups are supported consumer updates could add and remove groups. Today only the `PriorityTimeout` 
+could be updated.