Address PR feedback: clarify comment

Signed-off-by: Charles Korn <charles.korn@grafana.com>

promql/engine.go

@@ -1839,8 +1839,9 @@ func (ev *evaluator) rangeEvalTimestampFunctionOverVectorSelector(vs *parser.Vec
 
 	return ev.rangeEval(nil, func(v []parser.Value, _ [][]EvalSeriesHelper, enh *EvalNodeHelper) (Vector, storage.Warnings) {
 		if vs.Timestamp != nil {
-			// This is a special case only for "timestamp" since the offset
-			// needs to be adjusted for every point.
+			// This is a special case for "timestamp()" when the @ modifier is used, to ensure that
+			// we return a point for each time step in this case.
+			// See https://github.com/prometheus/prometheus/issues/8433.
 			vs.Offset = time.Duration(enh.Ts-*vs.Timestamp) * time.Millisecond
 		}