Add comment to explain why check if PV instanceof DisplayProvider see

pullrequest #3228

Author: katysaintin

app/pvtable/src/main/java/org/phoebus/applications/pvtable/model/PVTableItem.java

@@ -203,6 +203,9 @@ private void updateDescription() {
             VType currentValue = getValue();
             if(currentValue != null) {
                 PV thePV = pv.get();
+             // DisplayProvider is an optional interface for VType values,
+             // not PVs, but the custum datasource as Muscade happens to implement
+             // DisplayProvider for enum and bool PVs, so check for that here 
                 Display display =  thePV instanceof DisplayProvider ? ((DisplayProvider) thePV).getDisplay() : null;
                 display = display == null && currentValue instanceof DisplayProvider ? ((DisplayProvider) currentValue).getDisplay(): display;
                 if (display != null) {