clean up comments

Signed-off-by: Sam Alba <sam.alba@gmail.com>

dagger/inputs_scan.go

@@ -11,11 +11,13 @@ func isReference(val *compiler.Value) bool {
 	_, ref := val.ReferencePath()
 
 	if ref.String() == "" || val.Path().String() == ref.String() {
+		// not a reference
 		return false
 	}
 
 	for _, s := range ref.Selectors() {
 		if s.IsDefinition() {
+			// if we reference to a definition, we skip the check
 			return false
 		}
 	}

stdlib/dagger/dagger.cue

@@ -7,7 +7,6 @@ import (
 // An artifact such as source code checkout, container image, binary archive...
 // May be passed as user input, or computed by a buildkit pipeline
 
-// FIXME (perf). See https://github.com/dagger/dagger/issues/445
 #Artifact: {
 	@dagger(artifact)
 	#up: [...op.#Op]