Add comment for utility func

Signed-off-by: Tomasz Pietrek <tomasz@nats.io>

server/consumer.go

@@ -4600,6 +4600,8 @@ func subjectSliceEqual(slice1 []string, slice2 []string) bool {
 }
 
 // Utility for simpler if conditions in Consumer config checks.
+// In future iteration, we can immediately create `o.subjf` and
+// use it to validate things.
 func subjectFilters(filter string, filters []string) []string {
 	if len(filters) == 0 {
 		return []string{filter}