roachpb: clarify ResolveIntent.Poison

Specify that this field is ignored when resolving COMMITTED (or
otherwise non-ABORTED) intents (which it is). At lease the
rangefeedTxnPusher sets it for non-aborted transactions, leading to
question about whether that's a good idea.

Release note: None

pkg/kv/kvserver/intentresolver/intent_resolver.go

@@ -773,6 +773,10 @@ type ResolveOptions struct {
 	// ranges trying to read one of its old intents, the access will be trapped
 	// and the read will return an error, thus avoiding the read missing to see
 	// its own write.
+	//
+	// This field is ignored for intents that arent' resolved for an ABORTED txn;
+	// in other words only intents from ABORTED transactions ever poison the abort
+	// spans.
 	Poison bool
 	// The original transaction timestamp from the earliest txn epoch; if
 	// supplied, resolution of intent ranges can be optimized in some cases.