Update upstream.Lookup method comment to reflect current state (#4832)

Signed-off-by: Ondřej Benkovský <ondrej.benkovsky@jamf.com>

plugin/pkg/upstream/upstream.go

@@ -18,7 +18,9 @@ type Upstream struct{}
 // New creates a new Upstream to resolve names using the coredns process.
 func New() *Upstream { return &Upstream{} }
 
-// Lookup routes lookups to our selves or forward to a remote.
+// Lookup routes lookups to our selves to make it follow the plugin chain *again*, but with a (possibly) new query. As
+// we are doing the query against ourselves again, there is no actual new hop, as such RFC 6891 does not apply and we
+// need the EDNS0 option present in the *original* query to be present here too.
 func (u *Upstream) Lookup(ctx context.Context, state request.Request, name string, typ uint16) (*dns.Msg, error) {
 	server, ok := ctx.Value(dnsserver.Key{}).(*dnsserver.Server)
 	if !ok {